I've tried both, and the Synapse had a muted feel to me. The RS is a comfort bike as well, but you can still feel the road a bit more, which is not a bad thing. The RS also felt more responsive.

Having said that, I went with an S2 and have zero regrets. Just did a century on it this weekend, and although it's not a "comfort" bike, it was still comfortable enough.

Go with Cervelo. In the wise words of Ferris, "It is so choice. If you have the means, I highly recommend picking one up."