I think I like Big Ben a little more. Oakland's rush defense is abysmal which, may be exploited by Mendenhall, and in turn open up big plays for Wallace. I could be misreading this, and the Steelers will stay to the ground more, especially with Hines Ward questionable, but I don't think so.
Also, the Raiders are in the top few teams for sacks and Ben has sub-par ball protection. Did I just talk myself out of Big Ben? Maybe.
I think you're analysis of the way things will go in St. Louis is right, though. Too much running the ball, especially in the second half. And, unfortunately, they have a decent enough run game to run for first downs.