In some respects, Smash for Wii U is the same game released on the 3DS two months ago: it has the same characters, same premise, and same reverence for gaming history. The fundamental difference is in the depth of the experience.
The Wii U Smash has tighter controls, better action, more options for single- and multiplayer, more remixes of classic Nintendo songs, more stages, more customization, stage builders-the list goes on. The Wii U version is the definitive Smash Bros. experience.