Hard to say as their are many reasons for either reaching ten million first.
The PS Vita has currently sold more consoles
The WiiU has sold more per week on average than the PS Vita
The PS Vita has cross play with the new and very popular PS4 and may therefore spur on more console sales.
The WiiU has arguably better first party support for the console.
While I'm sure I've missed many reasons either way I am going with an indecisive 50/50.
Either way their both great consoles with great games.
This is the Game of Thrones
Where you either win
or you DIE