I don't know how much help I'll be, but my first bike was a CAAD9. I didn't really know anything else, except our Burley tandem, so I had nothing to compare it to. I just knew my husband told me CAAD frames had been used in the Tour de France and we got an insane deal. I knew I had a very nice "first bike".
When the SuperSix came out, I was drooling and REALLY wanted it. I was just biding my time, knowing there were no funds to get it anytime soon. And then my husband goes and crashes his bike and the next thing I know the owner of our LBS is dropping off a SuperSix with all my husband's old components on it.

DOE!!!
This did not please me. That is an understatement.
I avoided riding it, cause I just didn't want to know what I was missing. However, I went through a period where I was trying out some different saddles. After coming back from a ride on a new saddle, I wanted to compare it to a saddle I had tried and my husband had on his bike. So I hopped on and rode it around the neighborhood without giving it much thought.
Well, I did end up going with that saddle, but sadly I "accidentally" noticed a big difference in the feel of the bike. It was so smooooth.
Okay, so long story short, my husband surprised me several months later by taking my bike apart and putting it back together with a SuperSix and changing out my old DuraAce components for SRAM. The DuraAce was great, of course, but they were getting warn, as they had already been on his previous bike, and then mine, and plus I think he was having a lot of guilt and wanted to make my bike extra special and updated.
So while you wanted to know about the different between CAAD10 and Synapse 5, I can only tell you my experiences with CAAD9 and SuperSix. However, in looking up specs on both bikes, I did note that the CAAD10 has a full carbon fork (as did my CAAD9). It says the Synapse has "carbon blades". I'm not sure what that means, but the full carbon will likely provide more cushion, plus be a bit lighter..
My husband happened to walk in while I was typing this out. He looked at the specs with me of both bikes, including the type and thickness of the aluminum and type of seat stays, etc. In general, if there's not a huge price difference and you can afford it, he thinks the CAAD10 would be slightly more comfortable, though he doesn't see there being a big difference between the two based on the specs. He does think the CAAD10 would have a better resell value, with it being a racing bike.
I didn't look up the weight of each frame, but you might want to do that and see if there's any significant difference. I know my CAAD9 was pretty darn light.