WHM has 50 more potency than Benefic, Benefic 2, and Physick so an AST & SCH will have weaker heals than a WHM. Noct and SCH AoE shield are pretty much the same, their big difference is the power of aspected bene and crit adlo. If you feel like your heals aren't strong enough the you could try leveling as Noct to get the extra 15% boost to healing potency while in Noct stance. AST is more of a versatile utility healer so they can assume the shield healer or the throughput healer but they won't be king of it like their healer counterparts.