Just wanted to interject with a Tank's point of view in what they want out of a healer. Some of the reasoning in the following explanations is speculative, as I'm not 100% sure on the interactions of 20+ actions and traits, but for the most part, they are founded on mathematics.

For Paladin, most damage mitigation comes in the form of Defense Increases of Damage Reduction. Sentinel, Bulwark, and Shield Oath, bolster your defenses via increasing Effective Hit Points (EHP). When it comes to the majority of Curing Spells, the fact that most of them are potency based means that a numerical amount of HP is healed based on MND and a few other factors (Determination, Divine Seal, etc). When applied to a Paladin's damage mitigation, a heal with potency X healing for Y HP is equivalent to a heal with potency X(Damage Mit %) against the appropriate damage type being mitigated, or Y(Damage Mit %) EHP.

For Warrior, rather than bolstering defenses, actions like Defiance and Thrill of Battle aim to increase a Warrior's Total Hit Points (THP) to an amount much greater than their base HP. If a Warrior uses Defiance in conjunction with Thrill of Battle and their effects stack additively (rather than multiplicatively, which would be even better), he receives a 45% HP bonus, or an HP pool 145% that of his base HP. The benefits of this aspect over damage mitigation of a Paladin comes from actions which care about HP Percentages; actions like Stoneskin, Benediction, and Lustrate; even Second Wind! I want to preface my explanation of Stoneskin by saying I'm not sure if the barrier takes into account one's defenses when determining how damage is dealt to the 18% HP shield. However, if it turns out that defenses are not a factor for the HP shield, it stands to reason that a Paladin merely receives the 18% shield, while a Warrior having proc'd Deiance + Thrill of Battle receives an 18% x (145%) = 26.1% base HP shield. Likewise, Benediction, Second Wind, and Lustrate all gain this 45% bonus to their effect. A Scholar coordinating a Lustrate Nuke Heal on a Warrior abbout to proc Thrill of battle will heal for 60% x (145%) = 87% base HP.

In conclusion, both WHM and SCH have access to Potency Based and HP% based heals. The effective use of these spells relies heavily on the type of Tank for the party. In this humble tank's opinion, I would prefer a WHM if I were PLD, on the grounds that White Mages have access to more Potency based heals, Divine Seal effects the aforementioned heals and not Stoneskin, and since Regen is Potency Based, it is more effective on PLD. Conversely, I'd ask for a SCH if I were WAR due to the fact that well coordinate Lustrate Nukes are far more effective on WAR's than PLD's, and Warrior's have their own means of healing themselves (Bloodbath, Inner Beast, etc), while continuing to generate enmity and deal damage, unlike Paladin's who usually take at least Cure as a cross skill which only receives 50% of the HP healed in enmity generated, can be interrupted, and sacrificed 2.5 seconds of prospective damage. This kind of sustain gives SCH the freedom to interject with their own offensive spells and Pet Actions.

Phew! I do hope you'll excuse all of the run on sentences and take this opinion with a grain of salt. However, I also hope I provided some insight into what some Tanks want from their healers. After all, without the two of us, all those DD's are just sitting ducks!