For 56 cm frames, 2 cm of head tube height, 1 degree of head tube angle, 6 mm of fork rake, and 24 mm of wheelbase. Unless you're looking at the Tarmac SL 3, which loses another 2.5 mm of head tube. The Tarmac is not a harsh ride, it's just the kind of ride racers and sporty enthusiasts have been choosing for the last, say, 35 years.

My opinion is, if the Roubaix doesn't bite you on the butt and shout, "Buy me, I'm the bike you've been craving ever since your 40th birthday," get the Tarmac.