[I wonder if this is getting too technical for this blog.]
It seems very hard to evaluate the benefits of pixel density and photo site size. ("Photo site": each tiny light-recieving dot inside the camera. The larger it is, the more sensitive it is.)
One thing I noted in a professional review in a British mag, comparing the Nikon D3 with the Canon 1Ds III, was that it gave the overall image quality of the D3 to be at least equal to that of the 1Ds, despite the D3 nigh only half the resolution. (And of course you have smaller files and a more sensitive sensor.) The lenses could have something to do with it though.
My Canon Ixus 960, though, makes much sharper pictures than my old Fuji F10 (which I thought was wonderful), and doesn't have more noise despite having twice the megapixels. Of course, then the Canon is three years newer.
Given that there are so many factors, I wonder if we'll ever have a clear answer.