Depends! the wii u version perfoms better but the 3ds version has a larger active community.

As someone who plays both i gotta say... it depends. In singleplayer for fps 3ds sience it runs at 60. For comfort wii u, onlinewise 3ds because there are more 3ds than wiius, idk if it has crossplay, ik it does on local mode... Also there is an app to pass savedata to the wii u.

the bad thing is that the 3ds may have lagspikes if there's a lot of effects, which in wiiu, it does not. That might be a nono for some people so it's a thing to consider.