Fantasy football Running back help please?

I have Ricky williams, Who im pretty sure im going to start versus the Titans this week because the dolphins are fighting for the playoffs, Then i am wavering between Adrian Peterson of the Vikings and Cedric Benson of the Bengals, (I am leaning more towards Adrian Peterson because they are playing 5-8 Carolina but i am afraid that the vikings will rest their players against such an easy team) Then you have cedric benson who hasnt been so hot since his injury and is playing The san diego chargers who will give them a challenge. Suggestions on which two to start? please explain