As the true successor to the Wii console the Wii U will be a powerful system, from what we're hearing. It's been rumored that the Wii U will almost be as powerful the next PS4 and Xbox 720. But the Wii U is more of a dedicated console, not a handheld, so it's in a league that's a big step up from handhelds like the Vita. The Vita will probably be no where as near as powerful as the Wii U, but does pretty good as a handheld. I'm also guessing the Wii U will be somewhat expensive, my guess is around $350.
The Vita is a very good handheld, which competes with the 3DS. I like the Vita, but if I was into Nintendo I'd probably get a Wii U, as it's going to be so powerful and has a wide selection of first-party and third-party games coming for it. I really can't make the decision for you, you'll ultimately have to do that. I'm saying the Vita is good for a handheld while the Wii U will be good for a console (and you're into Nintendo).