Although Nintendo has yet to officially announce a release date for the Wii U version of Super Smash Bros., it seems that information regarding the game's release date has already been sent to retailers. This information has been leaked by one of these retailers, which gives us a possible release date for the title.
According to the marketing material featured in the photo below, Super Smash Bros. for Wii U will launch in North America on the 21st of November. If this date is accurate, this means that the Wii U version launches almost 2 months after the 3DS release.
At the moment, Nintendo has only confirmed that the Wii U version will launch sometime in Q4 2014. Meanwhile, the 3DS version is scheduled to release in North America and Europe on the 3rd of October.