It's fairly certain it will be the Wii U. Even if demand is through the roof for PS4 and NextBox, there is simply a limit to how quickly consoles can be manufactured, shipped and sold through the supply chain. The new systems will be happy if they sell as Wii U did last Christmas, which is very well and one of the best ever launches. Wii U, on the other hand, will have an abundance of systems available and likely some black Friday promotions and so forth. A repeat of last year's numbers would be considered a failure.
Further, the Wii U should have a large slate of available titles while selection on the new systems will most likely be limited and include a lot of games that are available on current gen. There is also a good chance that the systems will be $500 or more, limiting the audience and making the cheaper Wii U a more attractive option for many.
The only way the new systems would outsell Wii U is if the system really tanks and finds no interested customers. With new names and marketing I think this will be unlikely, and Nintendo still holds the desperate option of a price cut at this stage should things get real bad.
