Although it's the Wii U version of Super Smash Bros. we want most, the 3DS version is coming first on October 3, 2014. The Wii U game isn't dropping until later this holiday season. Why's that?
During a roundtable at E3 2014, director Masahiro Sakurai explained that the biggest reason for the lack of a simultaneous release is the debug process. For such big games, a large number of staff members are needed -- several hundred working on that aspect of development alone.
He also noted that both versions of Smash Bros. have a different architecture, meaning each requires complete attention during the debug process. Hence the staggered release schedule. The reasoning is about what you'd expect, but it doesn't make me feel any better. Oh well. Got to face reality sometime. (And I'm totally going to buy both, I just know it.)