The person you should see should at least have a Master's Degree in Psychology (LMHC, PsyD or PhD) or Social Work (MSW, LMSW or LICSW) but from there it is about the relationship. A social worker has little education in clinical psychology - an LMHC (licensed mental health counselor at the Masters level) has more, at least in Massachusetts. It really depends on their approach and what you are hoping to get out of the experience. I may be biased since I am going for my Masters in Psychology, but I think some one with an MSW can be quite good but doesn't have as much clinical knowledge as a Licensed Mental Health Counselor.

As for the PsyD and PhD's - they are generally on the academic/research path and may be quite knowledgable on the current literature. But that that has little bearing on how good of a therapist they are.