I have had both of these on my Six13 team. It originally came with the carbon crankset and due to a catastrophic failure the cannondale rep replaced my damaged frame and upgraded me to the Alloy SI crankset. There is a very noticeable difference in stiffness You also have the ability to switch out chainrings, I ride a compact on hilly rides and switch to standard on flatter rides, by swapping out the spider. with the carbon crankset you have to have two complete cranksets if you want to switch them around.