How do i explain to someone its not about megapixels ?

Whilst i'm after a Pentax K30 or one of the K5's and will use it for multitude of subjects she wants a 600D for the only reason of its got 18MP. and the only reason she is going for a Canon is cos its what everyone uses. so how do i explain to her its not just MP? i currently have a basic Oly CSC with only 12mp but its more than good enough for looking at the pics on a screen or printing at a reasonable size. she wants it for portraits so is never going to massive posters. i just don't know how to explain it to her.