I don't think it'll deflate Wii U hype too much:
- Amazing graphics
- Exclusive gamemodes
- New stages
- New controls (probably better)
- Unlock everything again
- Same screen multiplayer
- Probably more
As has been said, the developers' thoughts regarding Smash 3DS vs Smash U is that the former is more tailored towards allowing players to gather customisations, train their mechanics and get familiar with the characters and feel of the game in order to "prepare" for the more definitive (not saying that the 3DS version is inferior here) gameplay experience in the Wii U version.