I'd take westbrook because he's guaranteed to get the ball a number of times. Even though he's going up against the Pats, I think the best philosophy is to with proven point-getters and Westbrook is a much better fantasy back than McGahee in my opinion... but who knows maybe the league in which your playing has weird point schemes
(In one of my leagues a player is rewarded 1 point for every reception, so I start Westbrook regardless of what team philly plays because receivers get substantially more points than RB's based on how this league is setup)