Who should a I start at RB?

Clinton Portis against Philly or Chris Johnson against Baltimore. I'm starting Lynch in my other spot and Desean Jackson as my Flex. I want to go with Portis but I don't want to leave a game breaker like Johnson on the bench either. Both are playing tough defenses. Or I could slide Johnson into my flex spot and bench Desean Jackson......I've got to leave one guy out. Any advice on the situation??? Thanks in advance far any answers.