All of the healer girls are drops
Alarune and Archangel have a straight heal ability - Siren has an orb changer and her ability can be leveled by feeding her marine goblins, which are a pretty common drop, reducing the cooldown to a mere five turns. That's why she tends to be preferred.
Alarune has a stronger, longer cooldown skill, Archangel a weaker, faster one.
Otherwise, any of the three are equivalent in terms of being a heal-on-match party leader - only Amaterasu Okami, a rare machine only god has a better heal ability (they heal 3x recovery per match, Amaterasu heals 5x)