Divine Favor will always heal better. ALWAYS. The divine favor skill tree practically doubles the amount healed from the regular healing prayers, and in the same time, provide energy management (ex. Blessed Signet, Divine Spirit). While ritualists are good to have around, I will never put a ritualist in the place of a monk.

I played an assasin during FPE, and it was certainly appealing. However, an assasin's power is very limited. Their armor can only provide them with enough energy to burst a 4-combo move ONCE. After that, you need to wait for your energy to recharge to full before setting off another burst. Each burst can take out half of a caster's HP, but do very little against heavy armored mobs.