Diurnal is objectively better than White Mage at level 60 due to superior mana management, utility, a combined Asylum/Sacred Soil cooldown (which doesn't require people to stay put for more than 3 seconds) and the underestimated extensions offered by Time Dilation and especially Celestial Opposition. White Mage is only better than Astrologian at 50 because Gravity isn't available til 52. Without superior AoE damage, there's no contest.