I'd go with the Wii U for several reasons:
It's newer, so obviously it's being updated constantly.
It has online and it's free, so you can play with people on here or your friends.
Like I said, it's newer, so finding games won't be as hard.
The games are excellent.
You can purchase NES, SNES, Wii, and GameBoy Advance games off of the eShop.