If Palmer was for two weeks straight playing 2 of the worst pass defenses in the league and threw for 50 yards and 0 TD's and Losman the previous 7 weeks were playing the middle-harder pass defenses in the league and is averaging about 150 and a TD per game then yes I would start Losman over Palmer.
Except Willis's numbers are better and against harder defenses. Willis is averaging about 80 yards a game against the 3rd, 12th, 28th, 1st, 11th, 25th, and 3rd ranked rushing defenses in the league.
So lets recap:
McGahee: On average has played about the 12th ranked defense in the league all year long and is averaging 80 Yards Per Game with 3.8 YPC.
A-Train: On average has played on average about the 29th ranked defense all year long and is averaging 82 Yards Per Game with a 3.4 YPC.