I've got a Roubaix and a TriCross, and just helped a friend build a Crux Carbon. The Crux Carbon is about 1 lb. heavier than my Roubaix (56cm Crux vs. 61 cm Roubaix), both with Specialized S-Works cranks and Ultegra 6700.
Main differences: brakes and crankset. Road bike brakes are better. I use Avid Shorty Ultimates to get close to the feel, but the Ultegra brakes are slightly easier to modulate. Either one will stop the bike though. The Crux carbon has a 46/38 crankset, vs. 50/34 on the Roubaix. You can set up a cyclocross bike with road gears (I've done that to my TriCross) but then it's not as suitable for pure cyclocross.
One little nit on the Crux - only one water bottle cage.
Other than those, I don't think you'll find much difference betwen the plush road bike and the carbon cross bike.