The way I see it, I can imagine a summer release of Smash Bros for the Wii U (August maybe?) followed by a release of Smash Bros for the 3DS later in the year (October/November)
Reason being that 3DS has more third party support and as a result pushes out more titles, the 3DS version probably has more focus on single player elements (since it's a portable and not everyone will have wi-fi all the time), more focus on customization, and, like Pokemon Bank, Smash going wi-fi on the 3DS with such a huge install base (particularly in Japan) will be potentially heavy on their networks and they'll need some time to brace for that.
Also, there's a lot of people who now plan on buying only the 3DS version. Have Smash 4 out for a couple months, I bet many of them cave and get the Wii U so they don't fall behind competitively in the new metagame. It also prevents the 3DS version from stepping in the toes of the Wii U version ("MOM! I want the new Smash Bros.!" "OH, but it's so expensive ... here, what if you just get the cheaper version for your DS?")