It looks like owners of both the Wii U and Nintendo 3DS are going to have a lot of options for Super Smash Bros tools later this year. Early reports have uncovered an in-game option for syncing the Nintendo 3DS to your Wii U and being able to use it as a full-fledged controller for the console version.
Most serious players will likely look to a Pro Controller for their Super Smash Bros needs, but this could come in quite handy for those random moments a friend comes over and you don’t have an extra. More than just controlling your characters, the 3DS can also be used to transfer characters and information between the two versions of the game, so your stats and custom characters can go along with you if you decide to leave the house or play on a friend’s console.
Unfortunately it sounds like the controller functionality will only be possible if you own both versions of the game, so if you’re not planning on getting your mobile Smash Bros fix with the 3DS version then you’ll still need to use a Wii U gamepad or controller. Regardless, options are good and it sounds like there will be plenty of options for how to play it once the game launches.