The challenge for the Wii U version is that it has to not only sell itself, but it has to sell consoles.
The 3DS version will retail cheaper and it's for a system that's already in a lot of hands.
My personal prediction is that both versions will ultimately be the lowest-selling of the five Smash entries.
Smash 64 and Melee came out on systems that combined sold about as much as the 3DS has, however, those came out earlier in their system's lifespans, even Smash 64 by a few months. 3DS is aging but if it does sub-Smash 64 numbers that is very bad.
Smash U has to sell almost 1:2 on an eventual install base that may be as low as 15 million users to surpass Melee's 7.09 million on 21.74 million consoles, and that's a near launch game with an almost unheard of attach rate, about in line with the stellar numbers of N64 launch title Super Mario 64. Literally, Melee outsold Mario Kart: Double Dash, which admittedly came out 2 years later.
It'll be a small miracle if either, or both outsell Melee. I think that even the Wii U version can potentially outsell 64 though, if only just.
If both versions were able to outsell Melee, even if only just, that'd still be a huge sucess, even if it they can't touch Brawl.