What's the difference?

Regarding the quarterback derby, I'm told by people within the program the only real difference between Mark Sanchez and Mitch Mustain is that Mustain might hesitate on some plays. That is perfectly natural, because he's running the offense for the first time and in his first few practices.
Something I find interesting is that when a quarterback redshirts (at USC), it doesn't really help them much because they spend so much time on the scout team and so little time running the actual offense.
I guess my only question is with these factors, why not wait until training camp to choose a starter? Or does nothing change by then either?