Other pros and cons...

N810 has proper host mode (like the N8), N900 needs you to launch an app and enable it first, which is more of a faff.

Although N900 has better video capability and will play dvd quality video, I find that the upnp streaming from my media server (mythtv btw) sucks and after a minute it just buffers constantly, so any streaming needs to be converted down, which removes any advantage over the N810.

The N810 has windows file share discovery/browsing built in to the file manager, which is great for browsing and then copying files across. To be fair it does not like password protection. N900 does not have this and needs something like wizard mounter or similar.

The default media player on the N810 sucks - It's slow, and does not support albumart at all - alternatives are Canola, Quasar, and Mediabox

Yes the N810 is slower, but run it in performance mode, and avoid running too many apps together and flash.javascript heavy sites, and it's not far behind a stock N900 most of the time. My N900 gets bogged down as well from time to time.

N810s are seriously cheap on ebay etc for what they offer. Drop a 16gb or 32gb microsd card in, and it rocks.

If you don't need a physical keyboard, GPS, and will not be using it outside, then consider an N800 due to the 2 full size SDHC slots - you can then chuck 64GB in there.