Sure Sells Potential says that the 3DS version is going to have more sales than the Wii U. Now I think the Wii U is going to have a much larger percentage of system owners owning the game.
It makes sense since the 3DS has more hardware sold so therefore it has more potential consumers. However, the Wii U version has less requirements for multiplayer mataches. You only need one system, one game and 4 controllers vs needing 4 3DS systems and 4 games. If multiplayer is what people are after, then the Wii U will probably still be the way to go. And I have no idea how the internet connection is going to work on either. At times I get error messages just visiting people in Animal Crossing so I wonder how long a sustained internet connection will last on the 3DS vs the Wii U. Thus, I see why the focus is on enhancing the single-player aspects of the 3DS version as it will most likely be played that way for the majority of the time people use it.
It's interesting also how with these Smash fests going on, it is all focused on the Wii U. Now the Wii U needs the promotion but if the 3DS game is further along in completion since it will be released sooner, you'd think that could be demonstrated as well. In the end, I'm starting to think releasing the 3DS version ahead of time is a better decision as releasing it at the same time or after the Wii U version would probably lessen its sales and impact as it would be compared less favorably to its console counterpart. Moreover, it acts as sort of a trainer allowing those who want to get a headstart on the Wii U version more time to learn the new characters and moves and jump into the Wii U version with a bit more experience. Things are starting to get interesting as we approach the release of these games.