Which 2 running backs should I start?

I've got Frank Gore, Marshawn Lynch, Tim Hightower and Michael Pittman. Gore has a bye week, so I need to know who I should start, Hightower or Pittman? Also, Mawelde Moore, Kevin Smith, Warrick Dunn, Buckhalter, Faulk and Ward are all free agents. I've heard rumours Ward may start getting more carries? Who would you start and why? Im leaning towards Pittman.