more megapixels is never a good idea. Is bad for your noise and dynamic range, and colordepth, especially on canon 600D that have a smaller sensor then nikon D5100. I think 10% smaller sensor on canon plus 10% more pixels on canon give you 20% less quality on canon. It not about pixel quantity its about pixel quality. So that being said, nikon builds better cameras if it comes to pixel quality dynamic range, color depth and things like that.
Those 2 Megapixels wont make the difference on prints anyway.
I you want really big prints buy a hasselblad 60 megapixel midformat camera.
I am not saying you should NOT buy a canon I just say you should not buy it because of more megapixels. Just buy it because it feels better in your hands.