Should I drop/keep Lesean Mccoy for Cedric Benson, Knowshon, Hightower or Darren Mcfadden(ppr league)?

Current backs are Steph. Jackson, Brian Westbrook, Pierre Thomas and Lesean Mccoy (handcuff to Westbrook.) It's hard to keep Westbrook when I know one of these free agents will be a stud.