Thanks everyone for the great advice! I ended up test riding the Specialized Roubaix, the Trek Domane, the Cannondale Synapse, and the Giant Defy. From my test rides, I felt the latter two bikes did a decent job of smoothing out bumps in the seat area, but after riding over some rough roads I felt my hands go numb relatively quickly. The Roubaix and Domane had a similar feel in terms of comfort, but the Roubaix seemed to be a bit more lively when sprinting. I also liked the way Specialized put all compliance above the frame and kept the frame and fork rigid. Perhaps that what made sprinting on it so much more fun? It was also really hard to go with any other bike after getting used to Di2 during the test ride.

If it isn't obvious by now, I ended up going with the Roubaix in the end. And now for the obligatory new bike photo: