I'll get the 3DS version for two reasons: 1.) The cel-shaded graphics type adds charm 2.) I can play it anywhere I want, while I would have to be glued to my T.V. in the living room on the Wii U version
Smash Bros. is an essential living room experience with you and 3 of your friends duking it out. I would only get the 3ds version if I had multiple friends to play with, and even then it would be difficult to coordinate and lag kills a game like Smash.
I doubt it will be a problem. The original Wii wasn't as powerful as today's systems, and that the whole online thing was just a slapstick in the game. There's a reason why some single player features or similar modes are not in Super Smash Bros for Wii U and 3DS; they are making online improvements.