What do you think ? I think it would be a big mistake to release both versions to the same time. 

Prediction : 3DS version Spring 2015 

People would buy a WiiU for Super Smash Bros and later they would buy the 3ds version, because they want to play this game everywhere :) 

WiiU version for early November, 3DS version on Black Friday or the week after. They need to capitalize on that Christmas boost. Both versions should come out this year.

Simultaneous release. It'll be bad for the Wii U, but that's what most likely is happening.

Yea but I doubt Nintendo would do something like that and there are really good reasons to buy both... Different Stages is the big one and I am guessing there will probably be different Classic mode and Different Adventure mode for both as well... I think many people would prefer the wiiU version over the 3ds just because fighting games and consoles go hand in hand but the install base of the 3ds can't be denied.. I think it will do good for both consoles and its a win win for nintendo either ways but they do need to sell the wiiU more, thats for sure

The obvious option would be to release the Wii U version 2-3 months before the 3DS version to drive hardware sales, but knowing Nintendo I expect the opposite

Releasing them in different years would be the smartest option as that would give people no choice but to buy a WiiU this holiday if they want to play SSB. Of course this is Nintendo we're talking about so I'm sure they will release both versions at the same time.

I think they'll both release in Novemberish this year.

Aren't they releasing at the same time?