Hamister on 05 January 2014
I'd say it will be a close call this year. Wii U's chances to stay ahead i 2014 comes down to what impact Mario Kart 8 will have in the US. IF that game can be released early in Q2 and have legs to carry it and its hardware through the rest of the year, then I think Wii U will see quite a large sales boost well before XboxOne gets its holiday boost. That could very well mean Wii U can fend off XboxOne in 2014.
Eventually XboxOne will pass Wii U though, based on Microsoft's long term strategy for their plattform. A strategy I don't see in Nintendo's Wii U plattform.
