I'd say Wii U. Just for one reason and one reason only. The games. Lots of great multiplayer games like Nintendo Land, Wii Party U, New Super Mario Bros. U, Game and Wario, & Super Mario 3D World. Awesome single player games as well, such as Lego City Undercover, Pikmin 3, The Wonderful 101, Rayman Legends, & Zelda Wind Waker HD
I've had my Vita for a year now and only have Gravity Rush, Persona 4 Golden, Final Fantasy VII (PS1 classic), and the new game Tearaway. Not many games on Vita. I recommend waiting for a Vita until next year when Sony actually realize they have a failing handheld and make more games for it. They've been focusing a little too much on PS4 lately. Us Vita gamers have to wait until next year for more good games. :-/
Nintendo at least has Donkey Kong Country Tropical Freeze, Bayonetta 2, Monolith Soft's X, Yoshi's Island U, Super Smash Bros. U, Mario Kart 8, and the new Zelda coming out next year beginning Feb. 5th with Donkey Kong.