Originally Posted by El Diablo Rojo
They are both excellent racing bikes. Virtually the same geometry and overall stiffness. The Six13 is more compliant and heavier and of course more expensive to replace if you should bust it up in wreck. I've been racing on a CAAD8 for season and half and really like it. I find the frame to very comfortable, not just for an aluminum frame, but in general. I've ridden a Six13 back to back with my bike and it's definitely a smoother ride.
I haven't ridden the CAAD8, but I can tell you that the most surprising aspect of my Six13 when I first got it 18 months ago (2006 model) was the compliance of the aluminum rear triangle (identical to the CAAD8 from what I understand). I expected the carbon tubes to absorb road vibration, and they do. What amazes me to this day is how Cannondale can engineer the aluminum to ride like stiff Ti on the rear end.
I'm not sure how much heavier the Six13 may be than the CAAD8, but I doubt it's more than 150 grams or so. I'm still loving mine after 10K+ miles on it. I'm sure the CAAD bikes are wonderful bicycles. I'm pretty sold on Cannondale. Good company, great bikes. Zero bling. The perfect combination for me.