That's what we're looking for: Would you say the Synapse alloy is suitable for long rides, then, being more comfortable then the CAAD over distance? Or, would you skip the alloy versions, CAAD/Synapse, alltogether and go for the carbon option?
I will probably pick up a carbon Synapse or similar bike when I can afford one, but that is because I have a caad 9 and don't want two, somewhat similar alloy bikes. I think you can ride a good alloy bike over distance and everyone did before carbon became an option. If your budget allows, the carbon Synapse is probably the most comfortable (least vibration) option you've listed but that doen't mean it will be comfortable to you. I found the caad to be as comfortable as the alloy Synapse and maybe a bit more comfortable since it rode in a way that I preferred. When I can afford two bikes, however, I'm getting a carbon something.