From your description and choices I would vote for the Synapse Carbon 4. I have the Supersix 4...I LOVE it, but I do some competitive cycling. Were I not doing anything more aggressive I would stick with the Synapse, no question. I did my first century on my Supersix on Sat. and it was wonderful, but by the very end my neck and shoulders were a bit sore and tight from being in a relatively less upright position. For those long rides I think the Synapse is the more ideal bike.

The Rival components are great, too. Though if you like Shimano you may want to stick with Ultegra. I could never get the hang of Shimano shifters and have found SRAM to be much more intuitive for me.