1. To repeat what others have said, do NOT pay 1600 for the 105 caad 10. You should be able to get a 105 caad 12 for about that price.

2. The carbon synapse has better reviews than the Aluminum synapse. So while there's nothing really special about an aluminum synapse, there is something special, and substantially unique about the Aluminum Caad.

3. If you are already riding a 52 cm bike, I don't think you will have any trouble getting comfortable on a properly sized caad 12 (if you want to spend 1600), or a caad 10 (which is now old, and should be on sale for closer to 1300, imo.). The geometry differences will not be as big as going up 3-4 sizes.