Despite reasonable arguments to the contrary, there's still a very strong perception that more megapixels are better, even when comparing on the same size sensor.  It means that each individual photo receptor is smaller, and that means less light sensitive but the theory is that this is balanced out by increased detail and sharpness.  Canon, as of this article, currently rules the megapixel roost in 35mm full frame with the Canon 5Ds and Canon 5Dsr cameras.  The questions to ask are what does 50 megapixels bring you, and what do you give up?