If it were me I would keep Martin. I think Tampa would be more likely to focus on the ground game and be more conservative (less passing) in the red zone. If that's the case Martin should have more touchdowns than Ball. Yardage between the two backs is anyones guess. I would say that Martin will get more attempts but Ball's Yards per carry will probably be higer simply because with Manning in the backfield Ball will be running against more passing defensive formations.
It should be noted, though, that the last time I was in a Fantasy League I finished second to last, so take my advice with a grain of salt.
