I'm only getting the 3DS version first because it's out first. Had the two games been released on the same day, I would have bought both on Day One, but probably would play the Wii U version first.
I'm looking forward to the Wii U version more by principle, as we don't know much about it and Sakurai has said that both games are packed with content, so the Wii U version has to have a big new game mode or two in there. Plus, I can play with more people locally (because right now, the only person I can play locally with regularly is my brother on the 3DS. With the Wii U version, I can invite people over to play it even if they don't own the game). And while it's not a big deal, the Wii U version just looks more impressive graphically and will have an objectively more comfortable controller option.
In fact, part of me wishes the Wii U version came out in the Summer. Because I'll be back in college in the Winter. So even then, I'll probably end up on the 3DS version more as I'll bring it to college and play it when I've got nothing much to do, and I'll likely do work at home or something.