I prefer WHM in the earlier dungeons, because Scholar is boring when you only attack and EOS can heal everything. Also, enemies die so fast you can't do much with your DOTs. AST is also boring because you don't have the fun cards yet. It basically has the same abilities as the WHM.

Around level 40, I prefer Scholar. Around 50-60, I prefer AST.

I have always found WHM to be the easiest though if you want to get used to healing.