In my opinion, neither of them should be your main healer. PD has battlefield medicine, which is more of a Bleed/Blight dismissal ability than an actual heal, but it can be used to get people out of deathsdoor consistently, which is always appreciated

Occultist in the other hand can heal you for massive amounts, but he can also heal you for 0 and bleed you, essentially making him the only character capable of teamkilling someone at Death's door without being afflicted.

If you wanna play either of them as healers, I recommend bringing a second character with a heal, even if it's a small one. Occultist REALLY appreciates someone that can consistently and safely get people out of death's door so you don't risk killing them yourself. Bringing both PD and Occultist together can achieve that.