b,b, and pretty much a tossup but B.
While you might get the same number of innings out of all 6, those with greater stamina will pitch less innings with in-game fatigue. In other words, to keep a guy like 1b available to pitch without fatigue every 5th day, you'd have to set his pitch count below 95, meaning he won't be pitching as deep into games where in-game fatigue sets in.
Of course, the existance of in-game fatigue has not been expressly validated by admin, as best as I can remember. Please correct me if I'm wrong.