To clarify, do you mean a CAAD12? That would not be a good price for an older 10. I think you mean 12—I bought the same model but upgraded a lot of the components with what I had lying around.

I've ridden both, and honestly I don't really see a difference in terms of ride quality, but I'm no pro. Personally I'd go with the CAAD since it's the absolute pinnacle of engineering for aluminum and a wonderful frame to upgrade around. Do you mean the Synapse Carbon or alum?

And for the love of god get yourself a 58 or 60. Your back must be a pretzel.