I would go with Dwyer. Out of the four listed he is the only starting back. Mendenhall might cut into his carries, but the other three have to cut into other
guys' carries, if you get what I am saying.
I'm not thrilled about any of them, but if I have to go with a RB I will go with a starter. CIN is #9 against the run, but Jacquizz isn't getting the goal-line touches that Dwyer is, SD's backfield is now a three-headed monster, and Bell's change-of-pace usage is happening nowhere near the Red Zone, usually.