Marcum has a high effort delivery, Greinke doesn't seem to have one. I heard an interview with him once saying he isn't a max effort guy. He takes some off of most of his pitches to get more movement and better control. Add to that Marcum has proven to be more of a injury risk thus far, and I think it's safe to say Greinke's the safer bet moving forward by a wide margin. He had Tommy John surgery. That does not prove he is more of an injury risk moving forward. If anything it means less chance of having ligament problems in the future. Pitch type has been shown to matter. One of the worst pitches to throw is the slider. That was the stated reason Lincecum didn't throw them in spring or early in the season this year. It is also why Greinke stopped throwing them half way through his final season in KC. Marcum's repetoir is not as taxing on the arm as Greinke's.