Madrid is much closer to Ponferrada than Barcelona, so unless a flight to Barcelona is way cheaper than a flight to Madrid, I'd go with Madrid. Taking the train from Madrid up to Ponferrada will take you about 4.5 - 6 hours (depending on which train you choose) and a train from Barcelona will be more like 10 hours. You can check out train itineraries here:
www.renfe.com. They probably won't have itineraries published yet for September, but you can enter in sample dates in, say, April, and see what comes up just to give you an idea of what to expect.
You can also take a bus from Madrid to Ponferrada:
http://www.alsa.es/rutas/madrid-ponferrada.htm. Looks like it's only about 4.5 - 6 hours as well - not too shabby!
Ponferrada is a beautiful town and a great place to start. Buen Camino!!
Faith