i ride a cannon synapse carbon 2008 and i'm super happy with it.
what you'll notice from a carbon bike is certainly lightness...
as per stiffness and reactivity, it depends of the different builds and construction... so is difficult to say.

i know that the synapse is made for fondo-gran fondo distances and is very comfortable to ride on. it absorbs well vibrations, nothing to complain about.

what i really like about cannondale, as a add on, is the lifetime guarantee on the frame. should it break for some unknown reasons, they'll exchange it for free!

i advise that you test your bikes before and see how they feel. i would also call the manufacturing company and ask them about more details on the bikes you're interested in... you may get a more in depth insight and be able to make a decision more thoroughly.

hope this helps. it's exciting to buy a new bike, isn't it?