cowherd is rumored to be going to fox sports.
Not sure what Bill Simmons is famous for honestly. Never read his pieces or heard of him/them
He created this website called 'Grantland' which successfully blended sports and culture (read: the perfect demographic). He also helped produce ESPN's 30 for 30 documentaries.
I think he caught lightening in a bottle with Grantland. There are literally 1 billion sites which try and do the same thing, he was just lucky to be out a hair in front of the curve (with well-monied financiers).