I never touched the n810 so I might be a bit subjective but here are the reasons I went for the n800 when all three of them were available:

- It's the same hardware as the n810 - so I'll get more software updates (even if a used 770 would have been much cheaper)
- I get two sd card slots (much cheaper memory and much more memory) as opposed to the n810 which gives me 2 gb + micro
- I don't get the transreflective screen (that's ok - I use it outside only for music), if I browse or anything I'll be in the share, I don't get gps - that's ok too - it's a feature i want it in my phone not my tablet
- better speed, ram and more reliable hardware than the 770
- standard usb cable - easier to make a host cable
- The only point I gave a lot of thought was the keyboard. I like keyboards, I have one on my e61 phone and use it a lot. But I didn't thought in the end that it deserves the extra cash. I can always buy a bluetooth foldable keyboard and still be lower than the n810.

You could probably get a used one with all kind of accessories for around 200$ or a new one for the same price.

Good luck !