Full disclosure: I work at a shop that sells both Cannondale and Trek and am also in the process of trying to procure a Domane for myself.

The Domane is kind of awesome. I'm sure you've read enough glowing reviews, so there is no need for me to elaborate.

On the Cannondale, I love their bikes. I had a CAADX that I built up as a lovely commuter until I did something dumb and destroyed it and my ACL. The Evo is awesome and the CAAD10 is my favorite go to aluminum frame bike, even after Trek did a great job updating their alloy line this past year. But on the Synapse, I don't know. People really like them. Myself and a coworker rode it and felt like there was a weird point on the turn in where the fork suddenly wanted to sharply turn in (this was on either a 56 or 58, I forget). You wouldn't notice it in normal riding, only slow speed turnarounds. And the funky seatpost design is a pain to set with limited access to a screw that needs a very specific torque setting. It's a set and forget thing, and the proprietary seat cap on the Trek isn't any more "normal" but for me, I don't personally like it.

For sure, ride both and see what you like, but as long as you're asking I am partial to the Domane.

Oh and if you or anyone else here purchases a carbon Trek, be sure to ask for the included Torkey, a nifty tool with a 4 mm bit epoxied in that's set to a 5 nm torque. It comes with each bike, but they tend to end up in a box or floating around the shop because they are handy as hell.