Oh to answer your question, the Tarmac. Only because it is race oriented with a more aggressive geometry. This probably includes a shorter headtube, shorter wheelbase, steeper angles. This allows you to be more aero and turn faster.

However having a Roubaix may be better if you're doing a long ride or high climbing. The more relaxed geometry, will help you stay fresh and along with things like the Zertz inserts which will cut down on vibration.