Spielberg's films are so boring visually (with the exception of the occasional spectacle), that he might as well be working in the theater. Scorcese has a far better idea what the camera is capable of doing.
Spielberg may be a master storyteller, but his grasp of film as visual medium is no where near as sophisticated as someone like Scorcese. I just watched Casino on cable last night, I think it's a pretty boring formulaic mob movie, yet Scorcese does a brilliant job of telling a rather lame story...