I think De Palma is immensely overrated. Scarface was awful--I do plan on rewatching it before the weekend, incidentally, though. His best film is Casualties of War, and the casual Mission: Impossible close after--although granted, I haven't seen Carrie, or any of his earlier work.
Scorsese, on the other hand, is personally my favourite director, and for directorial vision, is possibly ranked among the all time greats. Taxi Driver, Mean Streets, Raging Bull, Goodfellas. All share similar themes, but it has worked time and time again for him--as a certain type of genre works well for certain authors.
Incidentally, Tarantino, on the 2-disc DVD of Pulp Fiction, on the Charlie Rose show interview, ranks both of these, along with Howard Hawks, in his top three most inspirational directors in his career.
Mick