IMHO, unless money is tight, I would not let a couple hundred dollars force me onto a bike that is significantly less comfortable. If they are both within your budget, I would ride them both and buy what is most comfortable. If they are equally comfortable/fun, buy the one that looks the best to you. You will ride the one you like the looks of more!
If you have a Giant dealer around, you may want to try the Defy, too. I rode the carbon Synapse, Roubaix and Defy Composite one day, and FOR ME, the Defy was the hands down winner. I'm not saying you will feel the same way, but I'd suggest you at least give it a try!