I bought my Wii U on day one, and I've never regretted it. The system already has some amazing titles out, with many more on the way. I'm not planning to buy a 3DS, so this is an easy choice for me. That said, I totally understand people who want to get their hands on the game asap or who want/need the portability that only the handheld version will offer. Others may prefer the 3DS version because of Smash Run, or they may not want to spend the money on a Wii U if they won't be playing many other games on it.
So, to answer your question the long way: you'll likely get a Smash experience that is "full enough" if you buy the 3DS version only. It's the same core game, so unless you need that big screen experience (like me), you should be just fine. However, it is my opinion that the Wii U is a ballin' system worthy of much more praise than it receives, and I would highly recommend buying it to anyone who wants the "full" Super Smash Bros. experience.