After 10 years in EMS I've started PA school so I'll add a little input here...
First off, PAs don't truly "specialize". You're trained as a generalist, with the large majority of schools focusing mainly on primary care. With that being said, there are a growing number of residencies available after you pass the PANCE to help you land a job in a specialty/sub-speciality. NPs are trained in a specific specialty, be it Family Practice Acute Care etc.
The idea that PAs end up with a better work week than MDs is a definite myth from practicing folks I've talked to/shadowed/learned from, especially as you enter the realm of specialties (Ortho/Cardiology etc.) In many offices (not all by any means) PAs are hired to perform more of the pre-op/post-op duties, as well as take call during the times that the MDs don't want to. Primary Care offices will often hire PAs and allow them to develop their own patient panel. From what I've seen the duties of the PA will depend on their SPs idea of how the relationship should work. Could end up great, could end up bad!
In regards to the educational models of NP v. PA there is a large disparity. NPs are trained in the nursing model, more of a holistic approach, whereas PAs are trained in the medical model. Many PA programs are run by the school of medicine associated with whatever university they are at, classes are shared with MS1-4s, same faculty teach many classes etc. So there is a large difference in the didactic period. Clinically PA students (from what I've seen) are exposed to a greater number of hours as compared to NP students.
A large point of contention between the two educations has been the fact that there are several online-only, and direct entry NP programs. PAs have always been proud that there has historically been a large number of direct HCE hours required for admission to their programs, but that is starting to go away with the large number of newer programs opening up.
A large number of states allow NPs to operate independently, that has caused quite the schism between the AMA and the BON. And although the opportunity is there to operate independently, from what I've read it's a statistically negligible number of NPs who actually do. PAs will always be required to have an SP, although state laws vary on what the duties of the SP shall be. Some require the SP to perform a certain % of chart reviews, some want the SP on site, while others require that your SP be available by phone in the incident a consult is needed.
And on a last note, while the terminal degree of the NP profession is indeed a PhD, that is an academic doctorate, and (in my personal opinion) should not be used as a title in the clinical environment. I find it to be misleading to patients, and false advertising if you will. PAs have the opportunity to earn a PhD as well, although if you have attended a B.S. program rather than an M.S. you will of course have to achieve your master's degree prior to attending. If I recall correctly there is the Doctorate of Physician Assistant Studies and a Doctorate of Clinical Health Sciences.
This post was in no way intended to offend any NPs or future NPs here, just my simple attempt to explain some of the differences between the professions. And now after looking at my watch, I realize that I have successfully slaughtered a large chunk of my study time...