Who should I start for week 15?

If I were you, I would go to the waiver wire and find a running back that will get a decent amount of touches. When you have a RB/WR flex spot, you should always try and get a RB into that slot, there will be more touches, period. If Percy Harvin is playing this week, you need to get him into your line-up as a WR and still fill your flex spot with a RB. Percy is a no brainer!