You have the difference in an aero aluminum frame vs. a carbon traditional frame. The aluminum will feel stiffer and jolt your body probably a little more than the carbon.
The Tarmac should have a D/A and Ultegra mix with the FSA carbon crank, the S1 has Ultegra SL and a FSA Gossamer crank.
I would go Tarmac, it has the better rear derailleur, the better crank and Kysrium wheels. I also wouldn’t want the compact crank. Seems like the Tarmac is a much better value for your money.