Well the Wii has half a year left on the market before Wii U comes out. So if you buy a Wii you won't want it for long. Though the Wii does have good games on their system and that's what it should come down to.
The 3DS has a long road ahead of itself being released earlier this year. But while the game drought of the 3DS is coming to an end, the drought hasn't quite ended yet. You'll have to be a bit patient with it for a few months considering there's a handful of big games on the market currently.
Seeing from those standpoints, I'd say get a 3DS. The Wii is at near death and it's not worth forking in 150 bucks for a system that will be discontinued soon when you have a system that'll be around for a while for $10 more.