The future of the Vita is very uncertain at this point, and it's a borderline flop. It's possible, even likely, that what little support there is for it now will soon dry out. Unless the games that are available or announced NOW are enough for you to buy the system, I'd stay cautious.
The U, though unreleased, has already outsold the Vita in pre-orders and has a good launch library, but still lacks AAA titles save the announced Smash Bros and Zelda games that we have no details on. I'd be cautious buying a launch, but I feel the U has a better future.
This is a sig. People who agree: 11. People who Disagree:4
The easiest thing i can say is to look at the game libraries that each system has and will be coming out with. I own a vita and can say that it's awesome. However that's just my opinion. I know that wii u will have backwards compatibility (you have to download an update for it) and you can download old classic games. PS Vita can play almost all old psp games(you would have to purchase or own digital copies of psp game), you can play select ps1 games(digital download), you have crossplay option, and soon they will let you use it as a controller to play some ps3 games. Each system has it's pluses so you just have to choose which library of games you like most.