Brawl started it's development at the end of 2005, and basically finished at the very end of 2007, which is why it first came out in January 2008 in Japan. If development of the next Smash Bros doesn't start until Spring 2012, and this one will be in HD with a much bigger graphical jump, the Wii U version isn't going to be ready until at last Fall 2014, more then likely delayed until Spring 2015.
Now the 3DS version I could see getting released earlier since it'll probably just port Brawls exact engine over to the 3DS. This right there would give them 39 characters already made that would be very easy to rework and edit, as well as a lot of other assets that are already done versus the Wii U version which will have to make a lot of it from scratch in order to be up to date with the Wii U's graphics.
So yeah, I'm expecting the 3DS version in late 2013 and the Wii U version in early 2015.