From what I understand, the ANCC certifies nurses in general from RNs to NPs in certain areas, advanced practice or not. The AANP only certifies NPs (ANP or FNP only) and represents NPs only. The problem with the AANP is that they test in cycles 3 times per year so if you don't know that (like me too late) then you have to wait until the next cycle, which can be 4 months. There test cycles seem to be heavily concentrated in the summer so if you graduate in August you better have everything ready to send.
The ANCC you can test all year so there are no deadlines, which is the only reason I am taking their test. I would rather take the test and support the AANP since their interest is more toward NPs but they don't have the convieince of the ANCC.
You also sign your name differently based on the test you take too, although I rarely see this done in practice most people sign FNP or NP not APRN-BC. If you are a student, I highly, highly recommend going to www.fhea.com
and look under certification information before you graduate. I also recommend, as do others, taking Fitzgeralds review course. It was better than my grad program.
I hope this helps.