My actual concern is that the 3ds version will hurt sales of the Wii U release - while looking at the more casual demographic that is. Many people own a 3ds without owning or having intent to own a Wii U,
many people. Had the Wii U version been released first, many more people would have felt inclined to purchase the console simply for the enjoyment of Smash Bros, which is a trend we saw on the Wii. With the 3ds version coming out half a year sooner, many of these potential buyers are instead going to get their "fix" of smash through the 3ds version, feeling less inclined towards buying the "same" game, a game they've already been playing for the last 5-6 months and have began to grow tired of. Ultimately, these people will then decide to just play the Wii U version with their friend that has a Wii U instead of buying their own console, causing the 3ds become the more personal version that everyone has, and the Wii U version being the version that only one person per friend group needs to have bought. I really hope this doesn't happen, but I feel like it will - many people hesitate to purchase a Wii U. That being said, the increase of purchases from Mario Kart 8 might motivate more buyers.
Let me emphasize that I'm talking about the extreme casual demographic, so I know most people here will buy both versions no matter what.