I have a 2015 super six ultegra and I love it. It's my best bike.
Gets used nearly every weekend for 40-50 mile rides with mates and 100 plus rides when time is available.
Initially I was going to go for the Synapse which is Cannondale's endurance bike but found that I preferred the supersix after test riding both. Rides beautifully and is very comfortable.
My advice would be to get a test ride on as many bikes as possible and go for the one that you like.
My first bike was red but was not all that, but maybe that was to do more with the 14" wheels.