Maybe I'm wrong but I'm still not sure that in an out and out sprint that Sagan can compete with Cav...yet. He is a very intelligent rider and he uses his strengths well, getting into a position ahead where any lead-out train for a rider like Cav would become obsolete.
I can't remember, for instance, which stage of the TDF but an incline towards the last few km of said stage, did for Cav. It didn't for Sagan and so although there was a sprint finish, Cav wasn't there to contend as he would be on a pure flat stage finish built for a bunch sprint. Griepel is stronger IMO whilst Sagan is a better all-rounder who I think will, also, excel in the Classics.