WHM is just raw power healing. It is super satisfying but less busy than AST.

AST is a very busy healer, which is actually why I like it cause I don't get bored on it. Diurnal AST is for the most part the same as WHM. You can have your AoE regen and your single target regen stacked and that is awesome.

Noct AST is a different character. However, you will see people saying "know when to place shields," but I live by the moto "why not just always have a shield on people if MP allows." There isn't any real reason not to always refresh shields.