The Nexus line traces its roots to developer-oriented devices, but since the LG-made Nexus 4, the lineup of Google-branded phones has been an affordable way to get great specs and a long period of timely software updates (even the Nexus 6 is a steal now).
The LG Nexus 5X is not spec'd like a flagship, like the 4, 5 and 6 before it were and the 6P is now. That's a letdown - the Nexus 5 cost $350 in 2013 dollars, the 5X costs $380 in 2015 dollars. And the price outside the US is just ridiculously high.
So, you're definitely paying more for the new one, what do you get? The screen is a definite improvement (its slightly bigger, with better colors and contrast), the chipset is faster for modern workloads, the camera is a major win and you get a new fingerprint sensor for faster, more secure phone lock. The much improved battery life (while not that great in general) is a big update over the old model too.
Google's motives with the Nexus 5X were clear. After Google Pay pretty much flopped, Google is now keen on pushing Android Pay, Google Photos and perhaps even Google Music. The fingerprint reader and camera really are the two biggest improvements over the 2013 Nexus 5.
It's not all good news though. For more money you get the same RAM and storage options, you lose OIS in the camera, the wireless charging and wired TV out. Subjectively, the phone isn't as attractive, objectively, the phone is definitely larger.
The Nexus 5X definitely lacks the cool, pioneering spirit of the 2013 Nexus 5. Not that some people would mind the "boring" life of quality hardware and reliable software. Geeks and power users, however, will certainly get a bigger rise out of the 6P.
It's been two years since the Nexus 5, right on time for an upgrade (even if you didn't get it on contract). You'll get less battery anxiety, your photos and videos will turn out much better, Android Pay (and other smartphone payment services are picking up speed).
You won't get more storage for your files though and the old Nexus is getting Android 6.0 Marshmallow too (probably its last major update).
The Nexus 6P is the first metal Nexus and it has a gorgeous 5.7" QHD AMOLED display. Looks aside, you get real stereo speakers, bigger battery, slightly faster chipset with more RAM, more storage (up to 128GB) and, well, a bigger price tag. Same camera and fingerprint readers though.
The HTC One A9 launched with Marshmallow and HTC vowed to bring new updates with near Nexus speeds. The phone has a lovely metal body (if reminiscent of an iPhone), an AMOLED screen and expandable storage.
The camera has OIS, though HTC's choice of chipset meant that 4K video is no go. Also the GPU is pretty weak and battery is small. Unlike most other phones on this list, this one has a fingerprint sensor (it's on the front).
Also, it used to be $400 in the US for the 3GB/32GB option and various promos occasionally bring it back to that level. Like the Nexus though, the prices outside the US much tougher to swallow.
The Motorola Moto G Turbo Edition updates a fan-favorite mid-ranger with a 5" 720p screen. It's water resistant (IP67) and a dual-SIM phone. The 13MP camera tops out at 1080p resolution though and the chipset is the same as the HTC. The second speaker grill is just decoration, but at least you get a microSD slot.
The OnePlus X also packs a 5" 1080p screen, but uses the old Snapdragon 801 chipset (almost from the Nexus 5 era). The X is a dual-SIM phone, but if you have only one contract you can put a microSD in the second slot. Do note that LTE compatibility with US carriers is a little spotty.
The camera situation is similar to the Moto above, 13MP with 1080p video though it did nothing to impress us.
The Oppo R5s has a 5.2" 1080p screen, an AMOLED one. The S615 chipset limits the 13MP camera to 1080p video resolution, but it comes with 3GB of RAM as standard. Also standard is the 32GB of storage (non-expandable).
Similar to the One A9, the R5s has a metal body, but this is ridiculously thin - a mere 4.9mm. Impressive as it is, the 2,000mAh battery is not.
The Nexus line is almost unmatched if you want new software. It was HTC that added the "almost" with their A9 Unlocked Edition, now that Motorola started dropping the ball.
LG nailed the screen and the camera and with Google's direction created an excellent upper mid-ranger. There's not a lot in its class that can top the Nexus 5X. This Nexus is less of the cutting-edge, dev-type of device we're hoping for and more of the polished, well balanced product that established manufacturers would gladly put forward. Come to think of it, we don't mind the trade-off as much as we thought we would. Now if only the price outside the US was more consumer-friendly, too...