If you haven't healed before and are deciding between those two, go with SCH. It'll be an easier learning curve, although between all three WHM is the easiest. Not to mention SCH also has the benefit of also letting you level SMN at the same time, so you can knock out two birds with one stone there. Also, both SCH and SMN are doing fairly well in this patch from what I've read (I haven't leveled mine past 50 yet), so that's always nice.