As well as running the same software, the Nexus 5X and HTC One A9 also have a number of other similarities, from fingerprint scanners to screen resolution, but there's a lot that's different too. To help you get your head around what's similar and what's not we've put the two phones head to head across all the key categories.
The HTC One A9 has a slim 7.26mm thick metal unibody, giving it high-end appeal. The bezels above and below the screen are on the large side, but with one housing a fingerprint scanner that's understandable.
The Nexus 5X looks less premium, thanks to a fairly plain polycarbonate back, but it does at least have a metal frame and it's fairly thin at 7.9mm thick.
The HTC One A9 arguably looks a bit too much like an iPhone 6S, but it's bound to turn heads, both because of those similarities and because of how stylish it looks in its own right. The Nexus 5X on the other hand is rather more understated.
Both phones have 1080 x 1920 screens, but the 5.2-inch Nexus 5X is a little bigger than the 5.0-inch HTC One A9. That's not a huge difference but could be enough to push you one way or the other.
If you want something compact the HTC One A9 is a better buy, while for more screen to play with you're better off with the Nexus 5X. The One A9 does have the advantage of using the brighter, more vibrant AMOLED technology though.
It's worth noting though that the Nexus 5X uses Gorilla Glass 3, while the HTC One A9 uses the newer and stronger Gorilla Glass 4.
Power and performance
The Nexus 5X has 2GB of RAM and a high-end hexa-core Snapdragon 808 chip under the hood. That processor has four cores clocked at 1.44GHz and two running at 1.82GHz.
The HTC One A9 has an octa-core chip, but it's a slightly slower Snapdragon 617, with four cores going at 1.5GHz and the other four at 1.2GHz. It has either 2GB of 3GB of RAM though, depending on which market you're in. For example, those in the US will get the 32GB, 3GB of RAM One A9, while those in the UK will have to make do with the 16GB, 2GB model.
Ultimately these are both pretty powerful smartphones, but both also fall slightly short of flagship specs.
There are differences though. The HTC One A9 has a microSD card slot with support for cards of up to 2TB, while the Nexus 5X has no slot, giving the One A9 a big bonus when it comes to storage potential.
The Nexus 5X has a 12.3MP camera, while the HTC One A9 has a 13MP one, so not a huge difference there, but the A9 benefits from optical image stabilisation, to keep your shots steady, while the Nexus 5X trumps it for video as it can record at up to 2160p, compared to just 1080p on the A9.
Round the front the Nexus 5X has a 5MP snapper and the HTC One A9 has an UltraPixel one, which is a fancy way of saying the pixels are bigger so it can let in more light.
Both handsets run Android Marshmallow, which is worth highlighting as that's the brand new version of Android and very few phones have it yet.
There are some differences though, as you get the stock version of Android with the Nexus 5X, while the HTC One A9 has HTC Sense on top, which is slick and stylish but not as minimalist as Google's approach.
The Nexus 5X is also likely to get prompt updates to new versions of Android, but there's no guarantee the HTC One A9 will.
The HTC One A9 and Nexus 5X both have fingerprint scanners, simplifying smartphone security and giving you the tools you need to make use of Android Pay.
But while the HTC One A9 has a scanner on the front the Nexus 5X has its scanner on the back. That's not a big deal but having it in front of you as on the A9 does make it easier to find.
There's a 2700 mAh battery in the Nexus 5X and a smaller 2150 mAh one in the HTC One A9. Google hasn't revealed any battery stats for the 5X and we're still putting it through our own tests, but the HTC One A9 can supposedly last for up to 12 hours of video playback, 16 hours of talk time or 9 hours of Wi-Fi use.
Those stats sound ok, but we don't expect this will be a phone that breaks the trend of daily charging. Fortunately both handsets do support fast charging and the Nexus 5X even has a USB Type-C port, allowing you to plug a charger in either way round.
Price and availability
The Nexus 5X can be yours for £339, US$379, which is reasonable value for money and massively undercuts most flagships. It also undercuts the HTC One A9, which comes in at £429.99, US$399. That's still cheaper than your average flagship, but not by much.
The HTC One A9 and Nexus 5X are both almost flagships. Almost, but not quite, thanks largely to less powerful innards and in the Nexus 5X's case an underwhelming shell.
But they're packed full of tech and features, such as fingerprint scanners and the latest version of Android.
The Nexus 5X also has USB Type-C, a bigger battery and the promise of regular Android updates, but the HTC One A9 has a more stylish design and a microSD card slot. So they have their differences but overall they're closely matched phones.
If you want a true flagship you could do worse than the HTC One M9.