Here's my take after owning both devices at the same time.
*Build quality- The build quality on the 800 to me was far superior than that of the 900. The 900 felt like my 710. The body felt more plastic even though they are both polycarbonate, and the texture felt cheap. Tapping on both devices with your fingernail made the 900 seemed cheap, while the 800 did not. The plastic bezel around the screen on the 900 does not help either, as dirt began being trapped between the body and screen and the bezel and the screen. Also the earpiece started accumulating dirt, since the unit was black, it was very visible! Not to mention the vibration motor buzzing the heck out of the camera lens, but on the 800 there is none of that.
*Screen- I am not sure whether it is due to the coating or the curved class, but the 800screen wins hands down. The 900 screen accumulating so much fingerprints and oily residue, it felt like my 710. When the fingerprints would "dry up" on the screen, it would have residue left behind which made the swiping experience very poor and the screen felt very gummed up. Also, as mentioned before, the cheap plastic bezel around the screen made dirt trap between the screen and the plastic bezel, and the plastic bezel and the body. With the 800, the screen was always very smooth no matter how many fingerprint smudges there were, it seemed less visible and the experience was always pleasing, not to mention that the screen flushed with the body so there was no dirt to be trapped between it and the screen.
*Scroll speed- The scroll speed also on the 900 lags compared to the 800. It's like as if the sensitivity was optimized for the smaller screen, but when put to a larger screen, it requires more movement of a swipe to get the desired scroll speed and distance. Top that off with the smudgy-gathering screen and the swiping experience can be a high-friction experience.
*Auto-brightness sensor- The sensor on the 900 is just plain stupid. It is a terrible experience. When you turn it on, you get a 10 second lag in the brightness adjusting to the light, and then it brightens up afterwards. When it scales down, it scales down in steps rather than smoothly dimming and smoothly brightening. It is annoying. Not to mention when using the device outdoors, it pushes brightness to high, which is fine, but then turn it off outdoors, go back indoors and turn it back on and its stuck to high, which would require turning off and on the device again to re-adjust to the inside brightness, and this is with the latest updates. With the 800, it scales smoothly, and there is no lag in adjustment.
On a sidenote, I got better battery life with the 800 than the 900. Both were configured the same way with the same amount of apps. I did not have LTE on the 900 since I am on StraightTalk. In the end, the front facing camera, bigger battery, bigger screen, and gyroscope I did not care for. The 800 wins hands down, and the 900, well it's just like the 710, garbage.