The basic difference between a PTO and a PTA is that PTA's are local units (individaual schools) connected to State PTA's , connected to the National PTA. As a PTA member you receive both the benefits and responsibilites of belonging to a national association. PTO's are individual self-governed units within local schools. They are not accountable to a parent organization nor are they tied to philosophies and programming that may not be in line with their individual priorities.
As a member of both a PTA and a PTO (Elementary PTA, Middle School PTO)I must say that my personal bias falls to the PTA. It has been my experience that PTA provides me with a broader perspective of education as a whole. I like the fact that PTA's focus is on advocacy. PTA has been instrumental in establishing many programs that we all benefit from including our nations first child labor laws, kindergarten programs, the School Lunch Program, mandatory seatbelts and child restraints, just to name a few. PTA chalanges me look beyond just what's good for my child or my individual school to what's best for all children in all schools and that benefits everyone.