I agree with Zamboni and Helenbike. The SystemSix is arguably the best frame Cannondale has ever made and 'Dale will never make a frame like it again -- i.e., combination of alloy and carbon. The CAAD9 (or its progeny) will always be available while the System will be harder to find in the coming years, especially if it's a new frame. I would seize the opportunity. The System is least as stiff as -- if not stiffer than -- the CAAD (I've ridden both extensively), a little bit more comfortable than the CAAD, and the System feels like you're on rails when you're descending because of the massive head tube and down tube. The frame weight I think is about the same as the CAAD9 (indeed, the rear triangle of the System is the CAAD9 rear triangle). What else? DiLuca won the 2007 Liege-Bastogne-Liege and the 2007 Giro d'Italia on a SystemSix -- I'm sure a little juice didn't hurt him either. Mauricio Soler took the KOM of the 2007 TdF on a System. And a retired Barloworld racer told me that the System was flat-out the best frame he's ever raced on.
Hope this helps.