Despite his lesser palmares now, I see WvA has more effective skillset and better circumstance to win more than Sagan. A far better TTist, a better climber, and to me is a better sprinter too. It will take more time for him to collect big wins, but I believe he will. The only things that can keep him is the other super talents, I don't think Sagan have half that many rivalling him in term of talents at his peak, that's another plus for WvA.
He is no match for Sagan when it comes to create excitement on the road, though, and it's likely not going to change anytime soon. So until WvA wins a ton more, I think Sagan will always be remembered as the "bigger" cyclist.