I road them both and the stability is wheelbase....I told you if you ride one size of roubaix down its the same as a tarmac...you can call it stability, but its wheelbase. The vibration dampening is real but seriously, if you are riding over crap roads your tires matter WAY more than your frame assuming bikes you are comparing are both carbon. If the Tarmac cant take 25c tires then ya thats a big difference, but little zerts inserts will not save your behind.

I ride about 200 miles a week between TT/road bikes and commuting. Pretty sure I get out enough to know the difference. The problem is when you get out enough you realize how small those differences actually are.