My husband has the Cannondale Supersix 105. He tested that against the Cannondale Carbon Synapse 105. He could not really tell the difference between the two regarding the "Plush Factor". But the Cannondale Carbon Synapse 105 is slightly more upright position (less aggressive) than the Cannondale Supersix 105.

He ended up with the 2012 Cannondale Supersix 105 because it was on sale (new last year) for $300 cheaper than the Cannondale Carbon Synapse 105 and also was cheaper than the Specialized/Giant bikes in the carbon bike with Shimano 105 category.