'Project Sora had intended to make a 3DS Smash Bros. once it had finished up a game on the system and had gotten used to the hardware's feature set. With the advent of the Wii U, though, we had a choice to make. Iwata asked us if wanted to make the next Smash Bros. on the Wii U or 3DS, and my thought was that we had to go on both platforms.
If we went solely for the Wii U, the HD graphics would really bump up the visual effects, but then we'd be stuck in another arms race. If we made this game another extension over previous one, we'd have to cut out the new things we could possibly do on the 3DS hardware and compete with ourselves again over the size of the character roster and the amount of gameplay we can put it. It wouldn't be a fruitful competition, but doing something completely new would be difficult for many reasons, not least of which that the gamers may not be satisfied with it. That's why we decided to think about ways to link the personal connection one has with his portable system to the gather-around-and-play aspect of console systems.'
What could be done on the 3DS that can't be done on Wii U other than 3D graphics? If anything it should be the other way around since 3DS carts have much less storage.