HTC has been gunning for Samsung's spot as the top Android smartphone vendor for the past few years.
Design and build
Galaxy S6: 143x71x6.8mm, 138g
One M9: 145x70x9.6mm, 157g
Samsung completely redesigned the Galaxy S6, giving it a mixed metal and Gorilla Glass chassis that makes it a claimed 50 percent tougher than competing metal handsets.
HTC chose not to radically rethink the One M9's design and instead merged what it claims are the best elements of the 2013 One and 2014 One M8 devices.
Being fully metal, however, means that the One M9 is 3mm fatter and 19g heavier than the Galaxy S6.
Galaxy S6: 5.1in, 2560x1440, 577ppi, Super Amoled
One M9: 5in, 1920x1080, 441ppi, Super LCD3
Samsung made a big song and dance about the Galaxy S6's screen, claiming that it can display 70 percent more pixels than the Galaxy S5.
This is interesting, but we've struggled to notice the difference in sharpness between displays once they break the 400ppi threshold.
For us it will come down to the battle between the Galaxy S6 and One M9's competing Super Amoled and Super LCD3 technologies.
Galaxy S6: Android 5.0 Lollipop with TouchWiz UI
One M9: Android 5.0 Lollipop with Sense UI
The Galaxy S6 and One M9 both come with customised versions of Google's latest Android 5.0 operating system. The Galaxy S6 adds Samsung's TouchWiz skin, while the One M9 adds HTC's Sense skin.
TouchWiz improves the Galaxy S6's business appeal with the addition of Knox and Samsung Pay. Knox is a security feature that lets companies create and manage a separate, password-protected and encrypted area on the Galaxy S6 and Galaxy S6 Edge. Samsung Pay is a contactless payment service based on NFC and MST.
HTC Sense, meanwhile, makes Android more intuitive to use via the inclusion of Sense Home. This adjusts the applications and services that appear on the home screen depending on the user's location. For example, when in the office it will push work applications, like email and calendar, to the front of the UI.
During our hands on we found the One M9's interface was noticeably more customised than HTC's previous One M8, a factor that could put some Android buyers.
Galaxy S6: Octa-core Exynos 7420
One M9: Quad-core 1.5GHz Qualcomm Snapdragon 810
We haven't had a chance to benchmark the Galaxy S6's "one of a kind" octa-core Exynos 7420, so can't comment on how it compares with the HTC One M9's Qualcomm Snapdragon 810 chip with 100 percent certainty just yet.
However, during our initial tests with both devices we were very impressed with both the Galaxy S6 and One M9's performance.
Powered by a 64-bit chip that is optimised for Android 5.0, the One M9 felt lightening fast and was noticeably smoother to use than HTC's 2014 One M8.
However, we were equally impressed with the Galaxy S6, which felt like one of the speediest handsets we've ever used.
Galaxy S6: 16MP rear, 5MP front
One M9: 20.7MP rear, 4MP Ultrapixel front
We were never big believers in HTC's Ultrapixel technology, which improves shutter speeds and low light performance by increasing the size of pixels captured by the sensor.
As a result we're glad to see that HTC has pushed it to the front of the One M9 and added a higher megapixel 20.7MP camera on the back.
However, as any photographer will tell you, picture quality is not necessarily determined by megapixel count. Loaded with a custom F-1.9 lens that Samsung claims lets in 60 percent more light than the Galaxy S5's lens, the Galaxy S6 may well outperform the HTC One M9 when it comes to low light performance.
Galaxy S6: Non-removable Li-Ion 2550mAh
One M9: Non-removable Li-Po 2840mAh
The One M9 has a larger battery than the Galaxy S6. However, Samsung has loaded the Galaxy S6 with a few custom charging technologies that have the potential to improve its usability.
This includes fast charge technology that Samsung claims can "charge four hours' worth of life in 10 minutes" and WPC and PMA wireless charging technology.
Storage and memory
Galaxy S6: 32GB, 64GB, 128GB, 3GB RAM
One M9: 32GB, microSD up to 128GB, 3GB RAM
Samsung is offering the Galaxy S6 with more internal storage options, although the One M9 is the only one of the two that can have its storage upgraded thanks to the microSD card slot.
Galaxy S6: Undisclosed
One M9: £580
The HTC One M9 is available to pre-order on the HTC website, priced at £580 SIM free. Samsung is yet to disclose the Galaxy S6 price. However Unlocked Mobiles is listing the 32GB Galaxy S6 with a £550 SIM-free price tag, indicating it may be cheaper than the One M9.
From a hardware perspective the HTC One M9 and Galaxy S6 are both top end smartphones.
However, of the two the Galaxy S6 on paper has a few differentiating factors that could attract users away from the One M9, which appears to be a refresh, rather than redesign of HTC's 2014 offering.
Key factors differentiating the Galaxy S6 include a "one of a kind" octa-core processor, updated Knox security, Samsung Pay service and super high-resolution Super Amoled display.
Check back with V3 later for full reviews of the HTC One M9 and Samsung Galaxy S6.
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