So far you've asked some basic but sensible questions. This one is just silly.

Take the specification sheet of the 50D and the 550D and compare them. The 50D is a low-end pro model which does everything better than the 550D.

The only thing where the 550D beats the 50D is the number of pixels. Tests I've seen between the 12Mpix Nikon D300s and the 18Mpix Canon 7D say that you have to make a very very big enlargement to see any difference between the two and that you need to have a very good lens (example 60mm Macro) and perfect technique (heavy tripod) to actually get full definition on the 18Mpix sensor. The difference between 15Mpix and 18Mpix is going to be even more difficult to see.

People who buy the 550D - consumers not experts - are going to be wowed by the 18Mpix but it's overkill for them (unless they crop a lot).