what's important: top tube length + seat tube angle + head tube angle.
the 50cm and 52cm of the caad9 aren't that different in terms of geometry.
The end result is a whooping 0.5cm 'cockpit reach' size difference between the 50cm and 52cm.

cockpit reach is nose of saddle to center of bar clamp length.


CAAD9 is more racing oriented and puts you in a more aggressive position
synapse is more recreation/comfort oriented (not that you can't do the opposite with either)