Personally I only own a 3ds so I can't really answer with much knowledge in the facts but my vote would be WiiU. It pretty much has everything that the 3ds one has plus (in my opinion) better controls (GCC FTW!), graphics, more modes, better online, etc. Although I do like the 3ds version, sometimes I feel like it's a bit lack luster.
I would give the 3DS version a rating of 8/10 and based on my current knowledge of the WiiU, I would give that version a 9/10. Both are great games but the WiiU version just has more.
I honestly don't care about the prize so can I opt out of it? seems like work :/