The choice is of course yours... Mer is supposed to be picking up to keep the N810 active and getting new software.. however I do not know what work has been done on it recently. It was usable last time I used it - however I removed the Mer interface and just ran a full debian+LXDE.

If you want to sell the N810 and get the N900, you will likely not be disappointed. Likewise, though, if you enjoy your N810 now theres no reason you can't keep it. There are still developers for the N810 and if you feel you are missing some software you can put up a request for it.

If you plan to just sell it.. well - some device is better than no device, is it not?