If you liked the Defy, I’d buy a Defy – but then I have one! The Synapse is more upright and you may not like that feel. The CAAD8 is closer in geometry (head tube length) to the Defy, believe it or not. Giant wheels have been much higher quality than Cannondale on our lower-end bikes (Avail and CAAD8). The Cannondale stock wheels were gash, if yor choice comes with Aksiums, that is a bonus, but Giant wheels are excellent (DT hubs and Giant make the alloy rims for DT).
Looking at all those options, the CAAD10 105 in a 58 is a bargain and the best alloy frame money can buy. Might be too big, and too low at the front, but it is an outstanding frame (from a paid up Giant fanbois). For your money I wouldn’t worry about groupset, but would go for carbon unless you know exactly what you want – you test rode a carbon Defy, If I recall?