the short version is both require prior medical background. all np's must have previously been nurses while pa's can be paramedics, nurses, resp. therapists, etc
both programs take about 2 yrs
laws are better in some states for pa's and some states for np's.
both pa's and np's write prescriptions.
pa programs typically have more clinical hours while np programs have more hours of "nursing theory" in class. np's and pa's working in the same setting typically have the same salary and scope of practice.
for general pa info see www.aapa.org and www.physicianassociate.com
and www.appap.org for info on optional pa residencies in almost every specialty.