Leiter and Isringhausen are both valuable pitchers, so this probably boils down to how you want to manage the games cap. Ordinarily, slowing your pace for starters would be a good idea, but this year, there's the distinct possibility of a strike or lockout. If play stops in August (for example), the team that has already used the largest portion of its allotment will most likely be in the best shape.
It's a tough call, and depends also on the scoring you use. Still, all things being even, I'd say go with Isringhausen. Your pace for starters is so high that a slight shift toward relievers makes sense.