Even though I had the 3DS first, and I really don't fit the requirements of this thread I do find a lot of the answers here weird.
I have 3 retail games and 6 PS1/PSP games downloaded on my PS Vita so far. The graphics of the PS Vita are obviously better than the 3DS, and has a more convenient way of multi-tasking. However, something that cannot be denied for people who truly own both systems is the fact that loading times on some Vita games are significantly longer than most 3DS games. It isn't a deal breaker, but I don't see anyone really talking about it much.
The PS Vita has an advantage in terms of shared accounts and pretty much wont have a problem with the amount of save slots each game has. However, that doesn't really change much for me personally as I usually only use one save slot anyway.
Personally, I think the 3DS has a much more varied and powerful library than the Vita in terms of retail games. When you combine the PSN content and the eShop content, the Vita gets much closer to the 3DS as far as games are concerned but the 3DS still wins in my opinion. But really, when someone says that the 3DS isn't as varied as the PS Vita simply doesn't know what games are on the 3DS.