As the others have said, the biggest difference is that the CAAD9 is slightly lighter and slightly stiffer than the CAAD8. In my short test rides, the CAAD9 definately didn't seem to flex as much at the bottom bracket (I'm 6'2" and 255lbs) and it seemed slightly more responsive (some might say twitchy?) while not being significantly more harsh.

Bottom line? You cannot go wrong with either one.