Originally Posted by
oldbobcat
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.
Unless you are riding some really rough roads or cobblestones, the ride quality is not going to be significantly different. And any difference can be negated by dropping your psi from 110 to 100 on the Tarmac. You don't even need to go with 25 tires.
The major difference is positioning. The Tarmac is much more aggressive than the Roubaix. This is why you would buy a Tarmac. If you are truly considering using spacers to get identical handlebar heights and position, don't bother buying the Tarmac.
Additionally, you may not be able to acheive the same position. Most manufacturers limit spacers to 45mm. If you are already near that max on your Roubaix, you will not be able to add another 20 mm (the difference in HT lengths).