I think it could go either way, both consoles are primarily made for core Nintendo franchises and when u take into account a few things it makes sense why Gamecube is doing better in the same time frame.
At this point Gamecube cost $150 for 9 months already vs Wii U at $300.
Gamecube had Smash Bros, Luigi's Mansion, Pikmin, Star Fox, Animal Crossing, Metroid, Zelda, 3D Mario
Wii U has 2D/3D Mario, Pikmin, Zelda remake
So Gamecube has the price and Nintendo game advantage currently so if Wii U gets its price down and starts releasing there core franchises at a faster pace than it may catch or surpass GC. The only problem is at the moment neither of those things seems to be happening. Wii U is sold at a loss at $300 so a price drop doesnt appear to be in the near future and Nintendo hasnt shown the ability to release games in a more timely fashion yet, 3 month gap between 3D Mario and Donkey Kong followed by another 3 month gap until Mario Kart.
When the herd loses its way, the shepard must kill the bull that leads them astray.
