The demo's had the opposite effect on me, it's making me lean towards not getting the 3DS version, at least not right away. That and the control options in the 3DS game are pretty limited and I can't change them to what I want. With so many unknowns about the Wii U version and how the two will interact, it seems best to wait and see.
With 50 GB to work with, the Wii U game should have all stages from the previous three games, but they're not that generous.