I don't think there is any difference in overall stability between the N800 and N810, but I've only used the N800.

I paid $400 for my N800 in January 2007; if I was getting a tablet now and had the same amount of money available I'd get the N810, mainly because of the keyboard. I have a bluetooth keyboard but it is always a hassle for me to use considering my chaotic lifestyle. I like the virtual keyboard just fine, but some applications (such as Labyrinth, a mind-mapping program) require a real keyboard. It would be nice to have a gps as part of the tablet -- I actually have two gps devices, which cost me about $70 each, but I don't always take them with me.

If you really want an FM radio on your tablet, you have to get the N800.

The N800 is just plain unusable outdoors under many conditions, so that's another argument for the N810.