Having ridden and owned both, I'll say there is a noticable difference between the two. The Caad8 is plenty stiff, but has a slight bit of flex over bumps and when putting out a hard effort. Caad9 has absolutely no whip to it at all, yet still feels comfortable over longer rides (I've been on a couple of 75 milers so far with it and felt fine). IMO, Caad9 makes even the six/13 feel like a wet noodle when you're out of the saddle.

That said, I'm not sure its worth the difference in price. There's a noticable difference in stiffness sure, but you really dont need it. Caad8 was raced in the protour very recently, and is still a world class frame.