I have 810 for some time already ... And WSOD'ed 770 ... 900 has too high price for me at the moment...

Only things I find better in 900 are:
1. Built in camera pointed AWAY, not in face
2. Twice as powerful CPU
3. Integrated GPS support

810 has camera, althou the front "glass" got scratched from active use so camera got useless... No way replacing it... But still, 810 HAS camera.
810 has ability to connect to mobile phones of Nokia, to use GSM features (althou haven't got any supported phone to test that), and GPRS features through BT... So, 810 ALMOST supports GPS thingy, externally...
So, only thing to consider as best update at 900, is CPU.

Althou, I haven't found any solution to sloooooow GPS on 810, and haven't found any possibility for localising to my country, I am still on 810, and will buy 900 only after a year about, when price will be lower and new software will be already avilable... And then, I will search some user reviews to make up my mind...
Maybe, after ruining my 810 maemo and putting "Mer" on it, it will be faster and better... but we will se...

PS: If anyone can help with my issues, please, do so...