This is why god invented the bet. If you really believed that there is in essence no difference in overall talent between Shields and Cain, then you should be taking bets to that effect, that Cain and Shields are expected to be on the mound a similar number of times when runs are scored.

But, if you believe that Cain is in fact more talented in getting positive outcomes when the ball is in play, and/or Cain is in fact more talented in stringing events to minimize their impact to run scoring, then you should be taking bets to that effect.