I don't have any experience with the Tarmac, but I do own a 2010 Roubaix Elite, which is one of the lower models. I've been EXTREMELY happy with the bike. I'm a Clyde and have found it very comfortable on long rides (I did my first century in over 20 years on it) and it's as fast as I can make it go...I'm sure under a stronger rider, it would be even faster!
When I tried it out at the LBS, the only thing I could think of to explain its comfort was "it's like riding my LaZy-Boy". I generally think unless you're looking to do "serious" racing that the Roubaix will meet all your need for speed, and still be comfortable for as long as ride as you want.
Charles