Well, it's partly a matter of opinion. First of all, while the GPS on the N810 can be useful, don't buy it if you're looking for a GPS unit; it works for some use cases, but if that's your purchasing motivation, you'll be disappointed.
As for miniSD, you won't find many of those these days - count on buying microSD cards that come with an adapter (e.g. all of them). This limits you to 16GB (at the moment) + 2GB internal, for the N810, but that's still a decent amount of storage. If you need more, go with the N800. There is also a third-party adapter kit that allows you to use a (one) full-size SD card with the N810, and while the kit is not terribly cheap, IIRC (link, someone?), the cards are cheaper, and currently larger in capacity.
As for price, I'm sure that you can order one from the US for cheaper than $400, especially if you get a deal.
In regards to BT keyboards, lots of people like them, but think about where you'll take your tablet, and how you'll carry it. If you're like me, and wear it everywhere except the shower and your bed, then the hardware keyboard is a must-have, as you can't always pack along a separate (larger) keyboard. If you often have a bag with you, and/or need to type documents, or a lot of e-mail, then a BT keyboard would be advantageous, but keep in mind that it'll work with either tablet.

So...
-SD card availability -> either
-SD card size/price -> N800 may be better
-GPS -> nice to have, but don't base your decision on it
-Keyboard -> if you do a lot of typing, buy a BT keyboard either way (N810's keyboard is decent, but it IS a thumb-board). If you want portability, then definitely go with the N810
-Price -> good luck finding a new N800, but probably a fair bit cheaper used than a new N810

Notes:
-the N800 has an FM radio, too, while the N810 doesn't. Probably not a big deal, but just FYI
-if you live in Canada, and order a US model, be forewarned that you're at the mercy of the paperwork-monkeys at the repair center if you send it in. I forget the whole rationale, but essentially, US models go to the US for repair, and Canadian models go to Canada. If, however, you happen to own a US model, and live in Canada, then they aren't really prepared to deal with that, and if you're unlucky, you may wind up footing the bill for duty at the border on the way back. This shouldn't happen, but it may. I talked to one of the higher-ups when this happened to me, and he got it sorted out, but it wasn't fun.