Tbh, you should try out all the heals (you need to level them sooner or later anyway in order to obtain the cross-class abilities).

In General one could split the heals into: pure healing and shielding. Pure Healing goes for WHM and AST, while shielding goes to SCH (yes AST has Nocturnal, but until they change something about it it's not worth comparing it to SCH).

One could also look for 2ndary stats, if you prefer crit, you could go SCH and AST (at least imo AST plays well with crit :X). If you rather like DTR & SS you should go WHM (which can be also played with crit, but most ppl tend to go to SS & DTR).