Defy or Synapse

I have an  aluminium synapse and I love it. It's perfectly balanced between fast and comfortable. Aluminium has a reputation for being 'harsh' but that's not true with the ALU synapse, I've spent 5-6 hours on it on countless occasions. I switch between a -6 stem with spacers under for long days in the mountains and a slammed -17 for shorter flat days. I have to use a different headset cap to get low enough though but that's a cheap fix. All in all it's a fantastic all-rounder. At the end of the day the Synapse and Defy are both cracking bikes, and I highly doubt you'd be disappointed with either. Pick the one YOU want the most. Don't let the opinions of others change your mind. If you can try swing a leg over both for a short while do it, it might be a hard choice but you shouldn't be disappointed, Cannondale and Giant make great bikes.

I slightly prefer the Synapse (to the Defy Advanced Pro 0) but I can't exactly say why. There's just something magical about the stiff yet shock-absorbing frame. Mine is slammed and for me it has a brilliant combination of shock absorption and yet feels sporty... especially down hills and country roads where it really excels. At the time I bought it (2015) there wasn't anything quite like it on the market in terms of my personal wants, but now we have bikes like the Focus Izalco Max it may well no longer be the best around for me. Who knows.

Note that I've ridden a Synapse considerably more than my short trial on the Defy and I've invested a lot of money in it so I'm almost certainly biased

I tried the carbon synapse (ultegra) and it was very light with a smooth ride but for some reason I didn't feel as comfortable on it as on the defy. To me the synapse felt more sporty than the defy but the defy was more relaxing. I don't know if it was the geometry or simply a differerent frame size but I do prefer the defy, which I eventually opted for (the di2 version)

one downside to the defy is that Giant use quite a few custom components (I'm thinking about the seat post and overdrive2 steerer in particular) which can make modifications more difficult

Synapse, really rather good.  

Never ridden Defy, have ridden a 2016 Synapse though, and its awesome.

Have not ridden a Synapse but my ali Defy 1 now 5 years old remains brilliant and my 1 year old carbon Defy Advanced with discs and 105 is superb. Great value, great company. Defy for me

Me too .

DEFY for me.