Differences equivalent to CoD DLC does not prevent two games from bein' the exact same game. Smash 3DS and Smash 4 are the exact same game down to their core and play the exact same way, with solely said minor, irrelevant differences differentiating 'em.
I do not think it sensible to release a multi-platform game to boost a struggling console's sales. Especially when the undoubtedly more successful 3DS version releases BEFORE the WiiU version, taking a further chunk of the profits away. And if the consumers end up not liking the 3DS release, why buy an entire non-portable console for a similar experience? One way around this is if they were indeed considerably different games. Complementary.
No, it'd be better for the WiiU. Not good for Ninty. The 3DS release is necessary for Smash to sell well, since it's virtually impossible that the WiiU version will push copies anywhere near Brawl's figures.
Also, for you, Bodine