I think that if everything else is equal the marginally bigger pixel size might offer slightly better dynamic range and tonal range but then again the image from the 1D MK III has to be enlarged slightly which might mean slightly less detail? To complicate things even more the concensus of opinion is that for tonal range and detail the Nikon D3 is just about the best there is although it 'only' has just over 12 mp. Perhaps Canon would like you to buy both and then you can tell us?
