There are some real tools in this thread.
Having said that, Hogan from my pov is going to be one of those guys that simply will put up solid numbers. He's got good size, speed, strength, route running ability and hands. Don't get me wrong, I believe we are grossly misusing Mike Williams, the guy should absolutely be used more often and in different situations and given the opportunity to do what he does best, which is to fight for the ball on fade patterns and back shoulder passes around the endzone.
I think they both bring different qualities to the table, I believe Mike Williams is better near the endzone and that Hogan is able to get himself open more often. All in all I don't believe there is much of a drop off one way or another, just happy to have both on the team.