Well, think about it. The two games are going to have two different play styles. No doubt the home console will be a style similar to the smash we already know. So why not get the Wii U version?
The 3DS version is still a bit hazy on how it is going to be like, but it is going to be a different experience. However, the way I remember Sakurai describing some of his ideas between the two is that they go hand in hand with one another.
So, I believe that if you're going to get the 3DS version of Smash, you are only getting a part of the whole experience. The Wii U version will most likely be the version that can act as a stand alone perfectly fine.
So, it is true that you don't need to get both versions, but I would find it to be a better experience for 3DS users if they get both.
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