Jemaz,
I'll agree that the curve ball works better more often than the change up. However that is only because it is easier to learn to throw an adequate curve than it is to throw an adequate change up.
A good change up is more effective than a good curve ball and contributes more to future success. It was kind of fun to listen to the other team's coaches telling the kids to stay back and wait for the "curve ball" and then to simply have my son bust the next one by them. No he wasn't throwing a curve. His change tumbles like a splitter and gets a nice sharp dip at the end.