I'm with Scorer...do you want to race or? If so, the CAAD9 is going to be a more stiff ride, and they are known for being great race bikes. I was on the verge of nabbing the CAAD9 Optimo 3 before I found my Fuji. Ultimately, I had no real interest in racing (other than occasional friendly runs), so I couldn't justify the sacrifice in comfort I would have made with the C'Dale.
You're basically looking at the differences between seat stays. Carbon (Felt)? Or not (Cannondale)?