I tend to prefer Priests since I usually get a Mediator to raise everyone's Faith to fairly high levels (but not to the point where they will leave, of course) while I'm saving JP for "Equip Gun". Chemists aren't that great on the healing end, since the most they can get is 150, but they are more reliable because they always hit, especially with Phoenix Downs, and have no charge time. So it depends on your play style, if you want stronger, slower healing get a priest with high Faith. If you want quicker, weaker healing, then a Chemist is for you. Frankly, I think Math Skills with White Magic mastered is the best way to go. Calculate Cure 4 or Raise 2 or Holy to great effect.