I've been bouncing between the two for weeks now. I have a pre-order on the Axon, but I'm really doubting myself on it. My biggest thing against the OP3 is lack of microSD. Yeah, the Axon will have better sound ... and that Daydream ready 2k display ... but beyond those two bits, how do you find they compare?
The Axon 7's OS Miflavor is not quite as polished as OP3 but it's not a major point. For example: setting a wallpaper or lock screen cannot be done from the gallery, it must be done from the Settings-> Wallpaper/Lockscreen. If you go through the gallery, it goes to crop but doesn't change the lock screen or wallpaper.
Also no camera short cut and the way to launch the camera from a locked screen is sluggish and slow.
The fact the Axon 7 doesn't have back lit dedicated touch buttons really bugs me. At night or in the dark, it's almost impossible to tell where the back button is, and where the home button is. The fact they're so close to each other means I often press home when I meant to go back. This is specially annoying in games.
Camera post processing is better on the OnePlus 3. The Op3 has a more snap it and forget it camera than the Axon 7. It took 2-3 redos on the Axon 7 to get the right shot. The Axon 7 if you tinker with it has a better chance to take better pictures.
As far as performance goes, both are pretty much neck and neck. No stutters or lag with either phone. Both work well for multitasking and are excellent for gaming. So far the 6gb of ram on the OP3 may have shown an advantage one in the entire time, and who really knows if it was an advantage.
Fingerprint sensors both work very quickly, and unlock the phones quick. The Op3 is just a hair tad bit faster and more reliable for me, however that is not an issue of the sensor but me not being used to a rear mounted fingerprint sensor.
Battery life... this is honestly the hardest part for me to judge because I changed my usage pattern. It seems the Axon 7 and the Op3 may have comparable battery life. If anything the Axon 7 may have a small advantage. I will have more info in the coming days.
I would make a point to pick which one is more aesthetically pleasing. Weigh in the camera /os vs speakers and display and make your decision. For me the OnePlus 3 won due to the soft gold touch, better build quality (buttons are solid an don't wobble like the Axon 7's) backlit buttons and physical home touch button, and better snap and go camera.
It was very close and quite frankly I think I'd be happy with either phone. Maybe if I can't sell my Axon 7 I will just keep it.