I own both,
Vita at (or close) launch
3DS this summer because of AC:NL and pokemon.
Vita is my main HH because of the easy multi-tasking, the badass screen and the sleep mode wich makes it very easy to hop into a game and back out.
Vita hardware puts 3DS hardware to shame, but 3DS software puts Vita software to shame.
That's my opinion. Not going to put a number here.
Edit: and the OS of the vita is also amazing!