Two different things, the N810 is my pocket/bed/sofa/car/gps/multimedia device. The Eee is my mobile work partner, I can carry it all day when visiting clients and I will not feel pain on my shoulder and neck after a long day of carrying it. Maybe I could replace the Eee using a bluetooth keyboard with my N810 but the extra inches of the monitor and the ethernet connection are very important for my job. Also having a full Linux distro on the Eee is more flexible than Maemo on the N810.
Both devices are good for mobility, but one is pocket size and the other one isn't.
So, I recommend buying the two devices :)