I leveled both WHM and SCH both to 60, while only used AST a few times, but in term of efficiency, AST rules in low level dungeons with "Essecensial Dignity" (somewhat equivalent to lustrate) and "Lightspeed" (reduces cast time by 2.5secs).. Those 2 skills are really come in handy in healing the tank even in the lowest hp possible in addition to AST cant almost never run out of MP in those level dungeons despite tanks messing up badly.. Then comes the draw card ability which buffs party.. Those are things that either WHM nor Scholar doesn't have in low level dungeons..

In level 60 dungeons, i find it that SCH is more dependable with lustrate and dot, making the damage comes surely while not ripping the enmity of inexperienced tanks.. WHM's playstyle sorta requires tank to be able to play decently which is somewhat sorta uncommon in random dungeons runs..

But in my experience overall, most players arent choosy over which healers serve the best in some specific dungeons as long as everyone's alive and well.. More commendations to heals who juggles DPSing as well..