I have neither platform, currently. As somebody who prefers handhelds to home-consoles I'd say the Wii U has the more diverse and interesting library for me. Vita has some interesting games as well, but I can't really think of a must-buy title for me whilst the Wii U has multiple ones which have been released/will release soon.
I'm interested: what are some good JRPG's on the Vita that aren't remakes/are unique? I own Persona 4 and FFX for PS2. I played the DS verison of Tales of Hearts (Japanese translation patch), so I'm not interested in experiencing the same story of what I consider a mediocre tales of game. I played Muramasa on Wii (and one can play it on Wii U.) The other games seem to be an Etrian Odyssey clone (which I get enough of on the 3DS) and a Monster Hunter clone (which I get enough of on the 3DS.) I think in the end while the Vita will have more high quality JRPG's and it is good for people who don't play too much of the genre, the Wii U will have more unique high quality JRPG's coming (XBC:X and Revelations: FE) which interest me. I hope to eventually get a Vita though! :)