Cannondale Synapse Carbon or CAAD12

FWIW: I was deciding between these two recently and went for the Synapse to be kinder to my lower back. I've just done my first couple of hundered miles on and am very pleased. The only thing I have to directly compare it to is my previous Allez which I would guess shares a geometry closer to the CAAD12.

Plusses - more comfortable, especially on my lower back.

Minuses - I'm not as confident in carving turns with it, which I suspect comes from the more upright  geometry.

Agree with all the comments. I've ridden both, Synapse is definitely the "all day in the saddle" bike, but it's no slouch either.

I've ridden both - the Caad12 105 (non-disc) and the Synapse Carbon Disc. Personally I preferred the synapse (I bought it in the end).   The Synapse has a very smooth ride, very comfortable all day machine,

The CAAD12 felt stiffer and a touch harsher, though not as harsh as some Alu frames from the past. 

Definitely try and ride both of them back to back and see how you get on with it.

I ride both. The main difference is body position as a result of the geometry as described above and there is less flex when accelerating on the caad12 (a very marginal difference). The synapse is a great bike for long days in the saddle. Horses for courses but I love riding both (not at the same time).

just get the synapse. alloy frames of today are not harsh at all, and nothing wrong with the comfort. but a cf frame will be more compliant and I'd bet, it is less prone to failure than a paper thin walled alloy frame.

Synapse, and, if you want to lower your front end, you can still get a -17degree stem and a low stack headset cover.

I think the seatstays and the layup of the fibres will provide a better comfort than with the caad12

Some of you guys really are suckers for marketing

CAAD12 geometry: http://www.7hundred.co.uk/images/2016%20Cannondale%20CAAD12%20Geometry.PNG

Synapse geometry http://epic-cycles.website/wp-content/uploads/2015/12/cann-synapse-carbo...

Taking the 54 of each as a middling size:

Model - stack - reach - headtube angle - wheelbase

CAAD12 - 55.1 - 38.7 - 72.9 - 98

Synapse - 57.0 - 37.8 - 72.0 - 99

So the CAAD is lower, has a shorter wheelbase but slacker headtube.  The Synapse is a bit more upright; interestingly it steers a little quicker but has a longer wheelbase so might feel direct while actually remaining more stable.

Bear in mind that both will come with 50mm of spaces so you could very likely get exactly the same fit on both - only a ride would tell you how vibration is dampened.

... do try other brands as well though. There's nothing stopping you going into an Evans or your performance LBS and seeing how they feel. Who knows - you may prefer a Focus or a Giant...

The Synapse is built to be longer and more upright through its entire geometry - they aren't a like for like match in terms of fit or performance and one can't be turned into the other.

If you ever plan on racing, get the CAAD12 but be prepared for a racier and more twitchy feel and a perceptible-at-first-but-you-get-used-to-it slight increase in harshness. And I mean slight because they both uses similar bendy-chainstays and bendy-seatpost tech. On the CAAD12 you soon forget you're riding on an ali bike... but you don't forget it's a racer.

If you plan not to get your back low and generally want something that's fast, smooth and confident for longer distances and rougher roads get the Synapse. Slamming the Synapse is not ideal and if it fits you when you're upright it's hard to later make it fit just right if you try to make it racy. It's a long bike so it's difficult to feel balanced and not stretched out when you have a high drop. At least I feel that way about mine. Your mileage may vary.

Both are awesome bikes and both will do you good but 1/ be honest to yourself about your usage and fitness 2/ try them both. Personally I'm on a Di2 Synapse Disc currently; it suits my needs and my shitty-roaded London KOM obsessions bang on but I'll probably switch to a pimped-out CAAD12 or a Focus Izalco Max as my flexiblity and fitness improves over the next couple of years.

Hope that helps.

The CAAD will have a racier, more aggressive position than the Synapse, and have (very) slightly faster/twitchier steering. Which you'd prefer will depend on you.

You could only ride all day or race on either bike if your body is fit enough to do so.
Taking maybe tyre width out of the equation, if they are different, here will be nothing to choose between them in terms of comfort or speed. They are road bikes at the end of the day.
Both bikes could be set up to replicate each other so you should choose the one you like the look of.

+1 for test ride.

You can 'compensate' for the positioning part of the geometry with tweaks to stem and seatpost, but the steering geometry will be different, so it will always feel different, and while it will be easy to make the CAAD12's front end higher, it's much harder to make the synapse's front end lower...

Only way is to ride them both. What's your current bike? Anybody could ride either but everyone will feel different about which is better for them. 

 Synapsis is more relaxed so taller head tube and I'd imagine shorter reach. The Caad is more agresive (same as my Supersix I think) so shorter head tube and slightly longer reach.  Then you add the difference in material and you've got even more differences.  

Personally I've not had any difference going from a relaxed geo to a more aggressive one but you may be different. I can ride my 6 all day and still be fine.