I chose both for now, although Wii U is finally gaining some momentum. I'm currently playing three games on each system: New Leaf, Disney Magical World, and Tomodachi Life on 3DS, and NES Remix Pack, Mario Kart 8, and Super Mario 3D World on Wii U.
I would really like to get my hands on a Wii U system. I tried playing the new Smash Bros. demo for 3DS, and I could not help but feel that the magic was lost because there wasn't any controller for me to use. In the near future I hope to acquire a Wii U along with a GameCube controller and play the heck out of that game!