Nintendo's American chief, Reggie Fils-Aime, has gone on-record on CNN to state that the Wii U versions of 3rd-party games like Call of Duty look 'dramatically better' on Wii U. It's the kind of claim we would have accepted some 12 months ago when we thought Wii U was going to be more powerful than Xbox 360/PS3, but Call of Duty: Black Ops II Wii U (opens in new tab) is already out in the US so we can see for ourselves. Consensus opinion is that it isn't better. Slightly different, yes, but certainly not 'dramatically better'.
Here's the interview in question (skip to 2:18 for the section in question):
Electronic Arts has at least gone on the record (opens in new tab) to say that FIFA on Wii U does look better thanks to the extra RAM etc, but we wouldn't say Call of Duty is in the same situation.
Source: CNN (opens in new tab)