I prefer Wii U. 3DS is nice portability, but graphics are hard to ignore on Wii U.
However, to "Counter-act" portability, since the Wii U gamepad is, essentially, mini-tv - you can play smash on Wii U on-the-go too ...you'll just need a power outlet.
And I'm certain that i recall some vehicles having a outlet ... unless you driving (no bueno)