the 3DS rocks, and has a ton of games on it. but my opinion is like, heavily biased due to me only having a 3DS. But I've heard the the Vita is pretty cool too.
Not sure how much my input really matters since I've only ever owned a PSP and 2DS but I'd say it depends on what you wanna do with it. If you're just looking to play / emulate 3DS/DS games then definitley go with 3DS. If you're looking for something more all-round or a hardcore system then I'd go with the Vita because of its size and better analog sticks. I'm not sure if its powerful enough to run 3DS games but the screen is still a limitation there anyway.