I have both and both are comfortable. Differences are pretty minor. I might have a bit more stack on the Synapse but it's not much. I switch between them without noticing much difference. Get the one you like and set it up how you like. If you are looking at an alloy Synapse, however, I might suggest going with the caad as I think it's a better frame. Shops usually have them set up pretty tall, however, so if it's uncomfortable on a test ride it won't get better. You, however, will get better. I've made mine more aggressive as I've gotten used to the bike (I'm 45 so not some young flexible punk).

If I think about it I'll grab a pic when I get home of the two so you can see the differences. Mine are 58s. How tall are you? I'm 6'3".