I don't know. I'm not familiar with Hans Zimmer. Philip Glass has done a lot of movies, and he's clearly the most famous living composer. His score for Secret Agent (a nineties adaptation of the Joseph Conrad novel with Bob Hoskins, Patricia Arquette, and Robin Williams) avoided Philip Glass cliches to a considerable extent.
I thought Elton John made the songs (circle of life, i just can't wait to be king,...) and Hans Zimmer made the music (this land, under the stars,...)
Anyway, I have no idea who the choose between Hans Zimmer and John Williams. I know the most about John williams' songs and I've got a lot of sheets of his music. But Hans Zimmer is also really good. (I love the music of pirates of the Caribbean 2, it's so great !)
John Williams score for Star Wars is considered timeless in the hearts of many. Those themes are recognized so easily throughout the world -- oh, and let's not forget his Memoirs of a Geisha soundtrack. Two completely different, yet amazing soundtracks.
John Williams and Hans Zimmer are both very talented composers. Unfortunately, I do not think they have girth to do others styles with the same strength. I do like listening to much of their music, but I also find it non-challenging and repetitive.
Other movies composers who I think write good music for the movies as the movie needs it are Danny Elfman, and Jim Fox.