If Robert Turbin starts, I'd go with him. Seattle is a run first team in a game they're bound to get ahead in. If Marshawn Lynch is a no go, he's a safe bet for 20 touches.
But if Lynch does go, I'd go with Michael Bush actually. Lynch is unlikely to get a lot of work, especially if the Seahawks get far ahead, but will do enough damage to limit Turbin's upside. While Bush probably won't get more than 8-10 touches. at least his role as a goal line back is defined.
In any case, I wouldn't start Marshawn. The Seattle/Arizona game would have to be awfully competitive for me to envision him getting many more than a dozen touches. While he could do some damage early, his upside to have a huge game is limited.