I always lean towards the extra pitcher, if only to keep my GOOD pitchers fresh for the closing run. While MIke is right about roster expansion, it also presumes that you're OK with throwing AAAA pitchers during the pennant race. If you have decent position players, you're almost always better off giving that 25th slot to an additional pitcher. If you have low STA/DUR or injury prone position players, well, then that's a different story.