Clement's only been good three years at Wrigley, a pretty pitcher friendly park. He's been mediocre to awful everywhere else he's pitched. He's no longer young, and, frankly, he's gotta touch of the Tomko in him, IMO. The guy always seems to flee somewhat from his potential.
Contreras is older, that's true. And he's an injury risk (the reasons for his bad seasons in 2003-2004.) Now, presumably, he's healthy. And he had a way better season than Clement last season. Plus, he's shown, like El Duque, he knows how to win when the games matter.