I have a Nexus 5x and just bought (but haven't received) a OnePlus 3. The 5x just has too much lag and stutters too much with basic things - things that my Nexus 5 (and even my Nexus 4) didn't seem to have problems with. The 6p is obviously a better performer than the 5x, but it's too big for me to want to keep. The OP3 may be too big as well, but here's how I decided that a non-Nexus would be OK for me:
Performance: The 5x just doens't perform well. The 6p is better, but the 810 isn't exactly new at this point, and it still has some serious thermal throttling issues. This is my overarching concern.
Screen: According to AnandTech, the 5x has a more accurate screen than the 6p, and the OP3 is considerably worse; however, when I see a photo on a friend's iPhone, I don't really care that the screen is much lower resolution. Color balance might tip it for me, though (though there, the 6p is better than the OP3 but not perfect.)
Build quality: 5x has essentially no real build quality other than 'it's plastic and doesn't fall apart.' By all reports, the 6p and OP3 both are well-made blocks of aluminum and glass.
Camera: The 5x/6p might have a technically better camera, but both are pretty good and will take pretty decent low-light photos. I've got a real camera with interchangable lenses if I really care about minute quality differences.
OS/Updates: I've only used stock Android on a Nexus for the past 5 years, but OP3 keeps it pretty close. OnePlus has been making the right noises about updates, but I don't know if they'll follow through, but what features are really on Android N that you're super excited about? Multiwindow is nice, but then you're going to be limited by the 6p's 810 and 3GB of RAM somewhat; I personally don't care about VR. Google has spent years ripping features out of the OS and putting them into apps that get updated via the Play Store - things like the Now launcher - and I really suspect that as much as a part of the nerd in me wants to be on Android N first, I won't really miss it for a few months.
Performance: The 5x just doens't perform well. The 6p is better, but the 810 isn't exactly new at this point, and it still has some serious thermal throttling issues. This is my overarching concern.
Screen: According to AnandTech, the 5x has a more accurate screen than the 6p, and the OP3 is considerably worse; however, when I see a photo on a friend's iPhone, I don't really care that the screen is much lower resolution. Color balance might tip it for me, though (though there, the 6p is better than the OP3 but not perfect.)
Build quality: 5x has essentially no real build quality other than 'it's plastic and doesn't fall apart.' By all reports, the 6p and OP3 both are well-made blocks of aluminum and glass.
Camera: The 5x/6p might have a technically better camera, but both are pretty good and will take pretty decent low-light photos. I've got a real camera with interchangable lenses if I really care about minute quality differences.
OS/Updates: I've only used stock Android on a Nexus for the past 5 years, but OP3 keeps it pretty close. OnePlus has been making the right noises about updates, but I don't know if they'll follow through, but what features are really on Android N that you're super excited about? Multiwindow is nice, but then you're going to be limited by the 6p's 810 and 3GB of RAM somewhat; I personally don't care about VR. Google has spent years ripping features out of the OS and putting them into apps that get updated via the Play Store - things like the Now launcher - and I really suspect that as much as a part of the nerd in me wants to be on Android N first, I won't really miss it for a few months.
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