There are a number of different types of treatment providers that can diagnose and treat PTSD.
Psychologists are people who have either a Ph.D. (Doctor of Philosophy) or a Psy.D (Doctor of Psychology) in a field such as clinical or counseling psychology. In some cases, a psychologist may have an Ed.D. (Doctor of Education).
To get a Ph.D., Psy.D., or Ed.D., a person must go to graduate school. Graduate programs in clinical or counseling psychology usually last about 5 to 6 years. During that time, psychologists receive extensive training in the assessment and treatment of various psychological disorders. Depending on the type of training received, each psychologist is going to have their own beliefs and ideas about the origins of psychological disorders and how to treat them. These beliefs and ideas are generally referred to as an "orientation."
In meeting with a psychologist, you want to first make sure he is licensed by the state in which he is practicing. You may also want to ask about his professional orientation. It can also be important to ask about his training and area of expertise.
Psychiatrists are people who have an M.D. (Doctor of Medicine) that have decided to specialize in the treatment of mental illness. Psychiatrists have been to medical school.
Like psychologists, psychiatrists also receive extensive training in the assessment and treatment of various psychological disorders. They are also going to have their own beliefs and ideas about the origins of psychological disorder and how to treat them. Many, although definitely not all, psychiatrists may follow a biological or disease model of mental illness. Unlike psychologists, psychiatrists are qualified to prescribe medication.
Other Mental Health Professionals
Some mental health professionals are not called "doctor." Some mental health professionals have a Masters degree in Social Work, Clinical Psychology, or Counseling Psychology. These individuals have also received training in the assessment and treatment of psychological disorders. As with other mental health professionals, you will want to ask about their professional orientation, training and expertise, and whether or not they are licensed to practice. Individuals with a Masters degree who are licensed to treat psychological disorders will often have "LICSW" follow their name. This stands for Licensed Clinical Social Worker.
So Who is Best?
There really is no good answer to this question. The mental health professional that is best is the one who best fits your needs. Ask a lot of questions. Make sure that you feel comfortable with the person you are meeting with. Do you agree with their treatment approach? Treatment requires sharing a lot of personal information with someone. Make sure that you trust and feel comfortable with the person you are meeting with.