I like Rosa better because if everyone else is dead/dying and the boss is about to do another crazily powerful attack you can have Rosa cast Life2/Cure4 and your party will be able to keep fighting and you can do that last 6000 damage.

If Rydia was in that same situation the best she could do for the party would be to cast Sylph or Drain, which only restore a small amount of HP. Everyone would be dead before she could finish casting any of her strong spells.