I do 40-50 mile rides on my caad9 without issue but not when I first bought it. It took time to build up some flexibility. I have a synapse too which is supposed to be more comfort oriented but I really don't feel much difference from the more agressive position of the caad. I have a 40 mile loop with 2,000 feet of climbing and use the caad for about 1 of every 3 rides.

That said, if you are looking for a long distance bike the Synapse is probably the better option in general but you'll need a few rides in to know for sure. I bought a Synapse because I thought it would be better suited to longer rides but really it was just a good excuse to add another bike