From a guide that I read, each healer is better at healing a certain type of tank. It went something like:
WHM heals for large amounts per heal, making them much better suited to heal a WAR tank without having to worry about too much overhealing, as WARs have a much larger HP-pool than a PLD, but less mitigation.
SCH makes a much better PLD healer, as their absorbtion effects attached to their heals allow a PLD to make the most of their low health and high mitigation.
FFXIV: Raxion Gunsoul - Brynhildr - 50PLD 50SMN 50SCH 50DRG
WoW: Raxion - Greymane- 90 Druid
FFXI: Nion - Asura - 99 BLU, PUP, SMN, DRG, PLD 50+ DNC, NIN, MNK, THF, WHM, SAM, WAR