The only downside to WHM is if you run with a drk. If the mobs die during/really quickly after Holy's stun has ended, there could be mp issues for the drk. So if I have drk I rather have sch healing and if I have pld/war whm. Sure it can be worked with whm+ drk but that requires you to have better communication than "hello" at the start of the DF expert roulet. But guess that's not to be considered as a scenario in this case. So if only basing it by the possible time of clear whm > sch > ast. Unless ast only pulls balances left right and center then it'd be whm > ast > sch.