IMO with current prices, no doubt the real deal is the N800.
The N810 has its own flaws regarding the design (light sensor position, dpad position) the camera is the same sxxt than the N800's, the gps isn't great and for serious typing you also should use a bt keyboard ...ah, and costs almost 2 X!
I have thinked in upgrade, but really no reason to do that. I'm going to keep my beloved N800 until real hardware upgrade occurs.