Andy_K 

This. It's actually kind of surprising that Sagan has been the guy challenging Gaviria. When guys like Griepel, Kittel, and Cavendish are on their game, Sagan only comes into play when something unexpected happens and he's the guy who is able to adapt or the finish isn't flat enough for the pure sprinters.

Gaviria is an interesting guy. He seems to have the top end speed of a pure sprinter but it looks like he can sustain the power longer than most of the other guys. I wonder if that is the reason guys like Kittel and Griepel have been out of the hunt. QS kicks it in early -- too early for the other sprinters to go -- and Gaviria just doesn't slow down.

As for Gaviria vs. Sagan, if you watched the Tour of California it was pretty clear that Gaviria is just a lot faster. The fact that Sagan has even made things close when Gaviria was in the sprint tells me Sagan actually has things dialed in pretty well.