SCH is harder to play due to it being a DPS/Support/Healer hybrid job (it's seriously a jack-of-all trades role), but that also makes it more rewarding because it brings an entire party makeup worth of skills to each battle (it can heal, buff, debuff, dps, mitigate, etc). However, it also has amazing MP recovery (20% MP every minute + additional skill for emergencies), instant heals (3 every 60s), and pet support, so it's a lot more forgiving of a job. Even a lackluster SCH can function in end-game with minimal difficulty, but will lack efficiency if its not timing mitigation with the fight's mechanics. A lackluster party won't have much of an effect on a SCH due to their greater MP regen and assistance healing by the fairy.
WHM is a pure healer with 90% of its toolset going towards aiding total healing potential, which makes it easier to play. Because of this, it's one of the most boring roles out there and many people avoid it in favor of SCH. On top of this, while the toolkit is simple, it has a much higher learning curve due to its lack of MP regeneration and massive heals which bring massive enmity. To make matters worse, its only means to mitigate enmity is tied to its MP regeneration, so a WHM has to constantly assess when he/she should use it because it has a 2 minute cooldown timer. (Is it more important for MP or enmity reduction at this point, and will I need it again within the 2 minute downtime?) This makes WHM a lot less forgiving of a job when mistakes are made, either by you or someone in your party. A lackluster WHM will cripple your party by keeping the BRD from using Foes or ripping mobs off the tank with excessive healing. A lackluster party can cripple a WHM by requiring them to waste precious MP on healing avoidable damage, or raising those KO'd.
To summarize the 2 roles
A SCH is proactive healer/mitigator that requires a different mindset other than "I see damage, I heal it."
A WHM is a reactive healer with longer cast times that must be compensated for.
A SCH has an advanced toolkit and brings a ton of utility to a party, but is more forgiving when mistakes are made.
A WHM has a simple toolkit and can't do much other than heal, but is less forgiving and requires a ton of patience.
A SCH is more of an independent healer, and can hold its own even with a bad party composition.
A WHM is very party-dependent, and can't function well without decent players around him/her.