Specialized Tarmac SL6 disc or comp is a great endurance road bike such as for millennium ride or tour racing hence why its called TARMAC; comfort for long distance racing because its carbon frame eliminate road buzz so you would able to seat longer for hours without fatigue.  It feels 'lively' when you ride on it...i;ve tested before during long distance group ride.

Cannondale CAAD 13 is build for sprint and crit like racing..a good all rounder race for an aluminum;  full power transfer to the rear wheel but not so comfort for long hours rides ...its closest rival to specialized with same capability is the Specialized Allez Sprint.

Just so you know, the Tarmac has an aggressive geometry (I think slightly more aggressive than Caad13). After all it has the same geometry as the S-Works Tarmac used by pro tour riders so I personally would not call it an endurance bike but I inclined to agree carbon frame generally offer better comfort vs Aluminium.