I agree with the last (but two) post. More MP are always going to be better. Just one proviso: provided its not at the cost of noise, DR or as Canon seem to be inflicting on us recently, low ISO banding.
More MP bring a major benefit not often discussed: The tonal range is better, simply because there are more pixels to represent the changes in tone. I know that I can get away with an A3+ print from an 8 MP uncropped RAW file on a good day, but it will never be as good as an 80 MP file printed to the same size. So just imagine how good an 800MP file would look.