One challenging thing about WHM is that you often have to start casting before people get hit because in some fights you have to move right after that or you have different priorities after that. Ultima HM is the best example here as you need to move to avoid things or pop next orbs or heal tank that is about to be hit by that greeny laser splash. I have little SCH experience though as I stopped leveling is as soon as I got e4e but I have an impression that precast and movement management is a bit more important to a WHM than to a SCH.
Generally SCH looks more interesting and challenging, but look at those heal crit numbers! HEAL POWAAAAH!
