I'm one of those people who got up early on that blustery Sunday November 18, 2012, drove myself to the nearest Wal-Mart, and stood around freezing with the other die-hard Nintendo fans waiting for the store to open so I could get my hands on their latest gem on its first sale date. The black 32 GB (seriously? 32 GB? There are iPods with more than that) Wii U. Since that day, I have felt slapped in the face by Nintendo multiple times for the many baffling business decisions they have made (NO games at release, waiting almost 2 whole years to start putting out their own brands and not just being a lesser Xbox 360 or PS3, and freakin' 32 GB of memory). Despite all this, I persevere in my support of Nintendo and their bizarre choices because, at the end of the day, they still make the best games around. /rant
On topic, though, I feel like these release dates were a good decision on their part, in the end. As many people have already said, MK8 will hopefully move some units, and releasing Sm4sh on Wii U at the same time would just be putting all their eggs in one basket. Another reason is that I suspect Nintendo is hoping that Sm4sh will be this year's Super Mario 3D World, which would explain them saving it for the holiday season. Finally, depending on how you read "Summer" and "Winter" release, it's possible that the two games could actually release as close as 2 months to one another. The last Friday before September 21st is September 19th, fast-forward 9 weeks and we're looking at November 21st (Super Mario 3D World was released on November 22nd, 2013). So the gap may not be that huge, yet it would still give the 3DS version time to make the big bucks for Nintendo from people who just can't wait.
As for me, I'm an iron-willed (somewhat apprehensively proud) Wii U owner, so I will only be getting the Wii U version of the game. None of my friends own a 3DS (neither do I), and I'm the only one of my friends with a Wii U at this point, so they can all come over to my place for local multiplayer, or I can just do "For Fun" online with randoms (praying....PRAYING that the online experience is good).