Eventually Wii U will get the upper hand, just because its sales should pick up more than Vita during the holidays. It's going to be interesting.
Wii U does not go well in the U.S. now, but wait till they get out more games like Mario Kart 8, the new Zelda, and the new FPS Miyamoto.
Look Vita in Japan at the beginning, they selling only 10-15 k on week, in 2012 in Christmas sold ONLY 20,000 units... and now sold 20-30 k on week.
And I will be the same with the Wii U, they not selling less of 25,000, and have an average of 30-40K on week in USA and during Christmas become a beast with 150-200k, maybe even 300,000 on week before Christmas.
Vita not could never do numbers like that in Japan ...