It's hard to predict how it is going to performing without knowing what games Nintendo will release in the launch window.
What if they release New Super Mario World, Wii U Fit and Wii U Sports before the end of the year? With only 3 titles the Wii U can outsell both HD consoles combined during the holiday season.
However if they decide to release only games like Pikmin, 3D Mario and Metroid Other M even the Wii can outsell it.