i spent a day test riding between the roubaix and tarmac... having a badly herniated disc in my lower back (and 4-5 other small ones) I went in completely convinced that the roubaix would hands down be my choice... I didn't even want to ride the tarmac frankly, but decided to just for comparison sake... I enjoyed the tarmac more, it may not have been the most comfy bike out there, but given my riding desires (recreational and fitness, no long 50+ mile rides) the tarmac was just so much more fun... it was a friggin rocket ship, every movement translated into forward motion... I felt like I had to push teh roubaix harder to get the same 'fun quotient' out of it... and I'm a heavier/older guy at 6-1 215lbs and 42yrs old with no modern road bike experience since the mid-late 80's...

ride em, take a 10-20 mile test ride, see which you enjoy most... if you can afford the carbon, get it... you won't regret it... just my .02