WHM is easier only because the class is simpler. PS3! PS3! Woop.
SCH is easier only because insta-cures 25% and constant Cure I / Embraces.

T1 - SCH
T2 - WHM
T4 - SCH
T5 - SCH

Garuda - SCH
Titan - WHM
Ifrit - WHM

L50 Dungeons - WHM (Holy spam for speed)

Anything which requires healing more than 3 players at a time regularly is preferred for WHM. Anything that doesn't, SCH is better with slightly higher single target heals.