You can't go wrong with either bike, they are both great. Nonetheless, the difference between the two lie mainly in the geometry of the frames.

The Synapse has a more relaxed geometry placing you in a less aggressive riding position, a bit more upright. The longer wheel base and the taller head tube and relaxed angles of the Synapse make it more "stable". The SuperSix has a race geometry, resulting on a more aggressive and responsive ride, which I think makes it more fun, potentially.

For reference I am 6'3" and ride a 61cm SuperSix, and I love it even if I don't race. IMO it is all about how you fit on the bike that makes it comfortable or not. Figure out the type of riding you want to do and decide from there.

Hey, they are both ridden in the pro tour by top cyclists. Like I said you can't go wrong.