I think it's a game that would be best to play on a handheld, it seems more convenient and sometimes I like to play in random spurts throughout the day
3DS definitely. Don't get me wrong, it would look amazing on the Wii U but I can't see Nintendo going to it for a main AC game. Animal Crossing was meant to be played with others and let's be real, online play would be pretty laggy on the Wii U. the 3DS is more convenient to play wherever you want and you wouldn't be restricted for stuff like Streetpass and whatnot like the Wii U would.
Unpopular opinion but I wouldn't mind seeing more side games on the Wii U.