You can make either bike feel like the other depending on the size you choose.....a smaller roubaix will feel a lot like a larger Tarmac....wheel base (center of gravity) and a taller head tube is what you get with a roubaix which means you wont need as many spacers and the steering is less twitchy.

Honestly, comfort bikes and performance bikes are the same if you outfit them the same. Especially when they are the same quality like the tarmac and roubaix are. If your saddle height, bar to saddle drop, and saddle angle are the same, its the same bike, if you can feel a crazy difference you are drinking koolaid