Special Master's Programs involve classes that build upon the prerequisites for medical school while post baccs usually involve the prerequisites. So for example, if you were a humanities major who didn't take Ochem and Gchem you would do a post bacc where as if you were a biology major who already fulfilled those req's you might look into an SMP (unless you want to retake those classes because your grades were too low). Also, an SMP would not factor into your undergrad gpa but a post bacc would.