Both the PRPC and the specialization exam are forms of professional certifications. You do not have to choose one over the other as they are two different distinctions (PRPC is developed and offered by H&W, the WCS was developed by the ABPTS). The PRPC is a certification of practical and didactic expertise pertaining to pelvic floor dysfunction. The specialization exam is a didactic exam that encompasses all aspects of women’s health physical therapy. You can be both PRPC and specialized in women’s health just like you can be a Certified Manual Therapist (MTC) and an Orthopedic Clinical Specialist (OCS).
Certifications offered by the APTA are only available to Physical Therapists, whereas Heramn & Wallace's PRPC certification will be available to the professionals qualified to take courses through the Institute.
For a full breakdown on the differences between PRPC and Specializations available from ABPTS, please see this comparison chart.