If they are going to have 2 different tanks in FFXIV then they need to be balanced, hp, damage done, damage mitigation, cooldowns. Currently they are not balanced at all. WoW has 4 different tanks, pladain, war, dk, druid. They all preform pretty similarly but what makes them different is there their attacks abilities, rotation, different buffs, debuffs they bring.

Classes can play differently which is the difference but still need to be balanced performance wise or one tank will always be preferred.