Until a couple of years ago, plastic smartphones were pretty much the norm from budget entry-level devices right up to £500 flagship handsets. Then HTC came in and blew everyone away with its all-metal One M7 , showing everyone just how gorgeous (and durable) a smartphone could be when its entire chassis was encased in aluminium. This was quickly followed by the equally good-looking One M8 , and HTC's brand-new One M9 is another stunning example

However, rival smartphone manufacturers are finally starting to catch up on the 'all-metal' trend, and no-one's taken this to heart more than Samsung, as it's released not one, but two all-metal smartphones this year in the form of the Galaxy S6 and Galaxy S6 Edge . Both of them are stunning smartphones - and, in our eyes, some of the best Samsung's ever made - but the big question on everybody's lips is simple: which one should you buy?

Some will have already made up their mind about which one they want, but if you can't decide which metal phone you want or just want some ammunition for the 'which is best' argument, then look no further. For the purposes of this article, we'll be looking at how the HTC One M9 compares with both the S6 Edge and the regular S6, just in case you don't want to pay the extra or simply think the curved edges look a little vulnerable, as both Samsung phones are largely identical apart from the screen. Now that we've had a chance to put both phones through our in-depth tests, we'll help you decide which handset should be the one taking pride of place in your pocket.

Design & Dimensions

Materials: Samsung has finally moved away from using plastic for its Galaxy handsets, for better and worse. On the plus side, the Galaxy S6 and S6 Edge are the best-looking, best-feeling S-series handsets yet. They both have a metal core, with a metal bezel, metal edges, and a glass rear that's bonded onto the chassis below. This gives it a slightly translucent, almost a jewel-like effect. Samsung's been a little vague on how this is achieved, but it's certainly impressive in the flesh. It comes in four different colours, most of which have a metallic-tinge to match the silver edges.

The S6 Edge takes this one step further with its front panel curving away gently at the sides to meet the metal edges. Whatever you think of the curved display's practical value, it's hugely impressive as a piece of engineering. It comes in a different range of four colours, with more grown-up, muted tones.

At first glance, the HTC One M9 appears largely similar to its predecessor, the HTC One M8. However, that's no bad thing given it was the best-looking Android handset of 2014 by quite a large margin thanks to its full-body metal chassis. There have been some notable changes though, including slightly more angular edges which give it a distinct lip, making the new handset much easier to hold. HTC has gone with a two-tone colour scheme, with grey, silver and gold in varying attractive combinations.

Coming down to the raw numbers, the Galaxy S6 measures 143x71x6.8mm while the Edge is marginally smaller at 142x70x7mm. However, once you've got both of them in your hand, the S6 Edge actually feels a lot thinner thanks to its tapered sides. We also found that the S6 Edge was much easier to grip thanks to its more angular frame. Both weigh 138g and 132g respectively.

The HTC One M9 is a little thicker at 145x70x9.6mm and weighs 157g, but is just as easy to hold as the S6 Edge. Again, the One M9's lipped edge provides plenty of purchase when you're using the phone single-handedly, and we actually preferred it to the smooth, rounded edges of the ordinary S6.

In short, Samsung has caught up with HTC in the design stakes, but it's really down to personal preference as to which handset you prefer. We're edging (excuse the pun) towards the Galaxy S6 Edge, as it not only looks and feels fantastic to hold, but its curved screens really draw your attention. It's a phone that you'll want to show off to people and feel proud to own. Even the iPhone users on the team are green with envy, so it's definitely got that 'wow' factor to carry it through the coming year.

Display

Screen size: The HTC One M9 has a 5in display, while both of the Galaxy S6 models use 5.1in screens. It's hard to notice the tiniest fraction of an inch in day-to-day use, but it's worth pointing out that the Edge variant is using part of its screen area for those curved sections, so you'll have to contend with reflections when watching videos and browsing the web.

Resolution: Both the Galaxy S6 models have huge 2,560x1,440 resolutions, which in theory (see below) equates to a pin-sharp pixel density of 577 pixels per inch. The HTC One M9 has a lesser 1,920x1,080 resolution, which is a more than adequate 441 pixels per inch. It's tough to see the difference in day to day web browsing, but app icons in Android were perhaps a touch sharper on the S6 handsets compared to the One M9. Either way, there's not much in and you certainly won't be missing out on overall picture clarity by choosing the One M9.

Screen technology: The reason those resolution figures don't quite tell the full story is because the S6 handsets and the M9 use very different screen technologies. Both S6 phones use Samsung's Super AMOLED displays, which have excellent contrast and really deep blacks, as each pixel illuminates individually rather than using a backlight. On the downside, Samsung doesn't tend to use a full RGB (red-green-blue) pixel structure for every quoted pixel in the spec, instead using a more complex arrangement of coloured sub-pixels. This means that the display may not be as sharp as its resolution might suggest.

The HTC One M9 uses a more-typical LCD, namely HTC's Super LCD 3 technology, which eliminates the air gap between the display element and the front glass. Contrast isn't as good as with an AMOLED but they are generally brighter at the maximum setting and each pixel does have a full array of RGB subpixels.

In practice, an AMOLED with good colour balance will outperform an LCD screen, and this certainly seems to be the case now that we've had a chance to test both handsets properly. Whereas our colour calibrator showed that the One M9 was only covering a measly 87.1% of the sRGB colour gamut, the S6 and S6 Edge hit the full 100%, producing much richer, more vibrant colours than the HTC.

Blacks were also much deeper on the S6 handsets, with the S6 scoring a perfect 0.00cd/m2 and the S6 Edge scoring a near-perfect 0.02cd/m2. The One M9, on the other hand, only measured a mediocre 0.35cd/m2. This means that text and black backgrounds won't be quite as dark as those on the S6 handsets, as they'll be shot through with shades of grey depending on how bright the screen is.

The One M9 does have a considerably brighter screen than the S6 handsets, too, as we measured a peak brightness of 478.50cd/m2. The S6 and S6 Edge, on the other hand, only measured 346.49cd/m2 and 341.09cd/m2 respectively. This is to be expected on an AMOLED screen, but it's still perfectly bright enough to use outside.

Performance

Processor and Graphics: Unlike last year's Galaxy S5, which was powered by a Qualcomm processor, the S6 and S6 Edge come with a Samsung-powered Exynos chipset. HTC, on the other hand, has stuck by Qualcomm. Despite the apparent difference, the two chipsets are actually remarkably similar, with both chipsets using eight-core setups, with ARM big.LITTLE architectures based around four Cortex-A53 cores for power efficiency and four Cortex-A57 cores for the heavy lifting.

The Samsung Exynos 7420's cores run at 1.5GHz and 2.1GHz respectively, while the HTC One M9's Qualcomm Snapdragon 810 runs at 1.5GHz and 2.0GHz.  The small clock speed difference aside, the real difference lies in the S6's Mali-T760 graphics and the HTC's Adreno 430. Both handsets include a whopping 3GB of RAM as well, which should be plenty.

Benchmarks: Now that we've had a chance to run our full benchmark suite on each handset, the Galaxy S6's Exynos 7420 looks to be much faster the HTC One M9's Qualcomm Snapdragon 810. In Geekbench 3, for instance, the One M9 scored a highly respectable 3,649 in the multicore test, but only 945 in the single core test.

The S6, on the other hand, sailed past both scores with respective scores of 1,427 in the single core test and 4,501 in the multicore test, while the S6 Edge was even faster with 1,496 in the single core test and 5,130 in multicore. This means that both S6 handsets will be much more capable of running several apps simultaneously, and will be quicker during less intensive tasks as well.

The offscreen Manhattan test in GFX Bench GL painted a similar picture. While the HTC One M9 managed a respectable 1,220 frames (or 20fps), the S6 produced 1,429 frames overall (around 23fps) and the S6 Edge was even smoother at 1,537 (or 25fps). We suspect the S6 Edge must be better at dissipating heat compared to its flat cousin, hence the faster scores, but in real terms, a difference of just 5fps between all three phones isn't going to make a huge amount of difference when you're gaming. The Manhattan test is extremely demanding, too, so all three are more than capable of running the very latest games from the Google Play Store.

Battery: We were surprised to discover that the S6 uses a smaller 2,550mAh battery, which is 250mAh less than last year's model. The S6 Edge, meanwhile, uses a 2,600mAh battery. Furthermore, neither of them are replaceable. While this is sure to upset some customers, the upside is that its chipset uses a 14nm process, which should make it more power efficient than before.

Sadly, the S6 and S6 Edge couldn't quite match the S5 in our continuous video playback test, as the S6 only managed 13h 37m when we set the brightness to 170cd/m2. The S6 Edge fared slightly better, lasting 15h 33m , but it's still a little disappointing nonetheless. Still, both vastly outperform the One M9's 2,840mAh battery, as this only lasted 9h 13m under the same conditions.

Storage

For years Samsung's Galaxy phones had microSD card slots, while HTC's largely didn't. Now it's the other way round, as HTC's kept the card slot from the One M8 while Samsung's dispensed with it altogether on the S6 and S6 Edge.

The result is that the HTC One M9 will be far more attractive to those who want a lot of storage, probably for music or video, while the S6 and S6 Edge come with a fixed amount of internal memory. The HTC One M9 comes with 32GB of storage, which can be expanded up to 128GB, while the S6 models have 32GB, 64GB and 128GB options (although the S6 Edge is only available in 64GB and 128GB options).

Camera

After last year's experiment with a 4-megapixel Ultrapixel camera, HTC has gone the other way for the M9. It's got a huge 20-megapixel resolution and a bright f/2.0 lens. The front camera uses that 4-megapixel Ultrapixel sensor, which should be ideal for video and selfies in challenging light conditions.

The Galaxy S6, in both its iterations, uses a 16-megapixel sensor, but has a slightly-brighter f/1.9 aperture lens. More importantly, it also includes optical image stabilisation, which should help considerably in low light by reducing camera shake at slow shutter speeds.

Despite having a higher resolution, HTC's camera definitely came out worst during our test shots. While colours were reasonably bright and vivid, darker areas of shadow simply weren't exposed correctly, resulting in incredibly dingy and dimly lit photographs. HDR mode helped to improve the situation, but this made other areas of the picture look even more unnatural than before, making some areas appear too bright and washed out to compensate the lighter areas of shadow.

^ You wouldn't know it, but this shot on the One M9 was taken at the same time as the one below on the Galaxy S6

The S6 and S6 Edge, however, produced much more balanced, accurate-looking pictures, and its HDR effect was much less noticeable. Switching HDR on did produce very slightly darker pictures overall, but it's barely noticeable unless you compare two shots of the same landscape side by side.

^ The Galaxy S6 lets in a small shaft of mid-afternoon sunshine, but it manages to expose the sky much more effectively than the One M9, and colours are much more accurate

Software

Both the HTC One M9 and Galaxy S6 will ship with Android 5.0 Lollipop , but the look and feel of each interface couldn't be more different. This is because each manufacturer like to customise Android to work better with their own apps and features, so the One M9 will ship with the latest version of HTC's Sense UI, Sense 7, while the S6 and S6 Edge will run the newest version of Samsung's TouchWiz interface.

Both versions of Android have received massive improvements since the last generation, and we particularly like what HTC's done with Sense 7. It's much more customisable than before, as its Theme Generator can change the icons, colour schemes and even the design of the menu settings to work with your chosen wallpaper.

^ We like how personal HTC's Sense 7 interface is as you can customise everything down to the caller ID logo

It also has a system that automatically promotes app shortcuts to your main homescreen based on your location, such as when you're at work, at home or on the move. When you're at home, for example, you'll see more media apps, whereas if you're at work, you'll see Google Drive, email and calendar apps. It works incredibly well, and we definitely prefer it to Sense 6.

Samsung has a new version of its TouchWiz interface too. It claims to have reduced that amount of widget clutter and pre-installed apps by 40%, which is a relief as previous phones were stacked with stuff we never used. Indeed, there were only two pages of apps in the app tray when we first turned on each S6 handset, and even then most of them were download shortcuts so you could choose whether to install them or not.

^ Samsung's TouchWiz interface hasn't changed a lot since the previous version, but it now looks much cleaner with fewer apps and widgets clogging up your home screen

One thing that has been added is a host of Microsoft apps, with Samsung having cut some kind of new deal with the company – maybe it doesn't like how beholden it's to Google for its OS. The Edge has some extra features of course, with information and contact shortcuts appearing in the curved sides of the screen.

Price & Verdict

All three handsets are extremely expensive, but the HTC One M9 comes out as the most wallet-friendly, with SIM-free prices currently hovering around £570 compared to the S6 's £600 for the 32GB version and the S6 Edge 's eye-wateringly high £670 for the standard 64GB version.

On contract, the S6 and One M9 have very similar prices, with the One M9 coming out slightly cheaper depending on which deal you choose. At Carphone Warehouse, for instance, you can get a One M9 with 5GB of 4G data and unlimited calls and texts for £39-a-month with an upfront cost of £29.99 (taking the total cost of ownership over two years to £966), whereas the same deal on the S6 costs £44-per-month with no upfront (which would cost £1,056 over two years).

The S6 Edge, however, attracts a much higher premium, not least because it has double the amount of storage. Here, the same deal costs £53-per-month with an upfront cost of £79.99, taking the total cost of ownership to £1,352 over two years. Still, when the S6 Edge is easily the most attractive and desirable-looking phone of the lot, we're not sure these price differences will be big enough to persuade you to buy one over the other if you've got your heart set on a particular handset.

Adding up everything we've gleaned so far, from the specifications and our own hands-on opinions, we think the Galaxy S6 Edge, or the flat S6 if you prefer, has the advantage over the HTC One M9. It's a closely tied match, but right now we're confident about which one we'd rather have in our pocket, and it's the Galaxy S6 Edge.