They're both nice carbon frames, and given the same tires, the ride quality over various road surfaces is likely similar between the two. My Roubaix allows 25 mm tires, which I can run at lower psi than 23 mm. If it's ride quality over chip seal that's important to you, that might sway the deal, if the Tarmac is limited to 23 mm tires.

I got the Roubaix for my back. For more sporting riders, it didn't look like Contador and others had any trouble in last year's tour riding Roubaix/S-Works frames quickly on the cobblestone stages.