They're both great bikes. I have ridden both and would be happy with both. With that being said I'd probably go with the CAAD9-5. You're getting an amazing aluminum frame vs a decent carbon frame. I ride a Cannondale fixed gear where the frame is very similar to the CAAD9 and I've done some 40-50 mile rides on it without any comfort issues. Just make sure your fit is right and if you feel like being a little more upright, flip that stem up. Obviously the carbon frame will absorb a little more road buzz, but they're both plenty stiff, agile bikes. If you're thinking that the Six Carbon will be some featherweight ride, think again, it is almost identical in weight to the CAAD9. Both equipped with 105 which will perform just great. I was leaning towards the Six Carbon 3 just because I knew I would eventually want a carbon frame, but found a deal on a Trek Madone that I couldn't pass up. Also, Cannondale is rolling out the 2010 SuperSix so you could probably find some sweet deals on last years models. The SuperSix is my favorite of the Cannondale lineup. If you're not trying to race (I'm not) and the the price difference is small, you might find a bit more comfort through the absorption of carbon, but the six carbon is a bit more sluggish feeling than the CAAD9. Sleep on it and give it one last test ride.