Two cameras each having the same number of megapixels may not be comparable at all of image quality. Franco speaks the truth. Some of the updates have been in the image processor. How it handles noise etc. Also, the newer cameras are often faster to turn and be ready to shoot and will shoot more frames per second.
The bottom line, in my opinion, if you're happy with what it does why would spend more money? Unless of course the answer is, "ya, I'm happy, but I wonder if I could be more happy with a newer camera." Only you can really answer how much good images are worth to you.
I love my cameras and frankly end up with more gear than I need, so this is just my two cents worth.