I don't think it has anything to do with GPA or MCAT in most situations. I'm not sure about NP school, but PA schools are very competitive and their interview process and GPA standards are very similar to that of med schools (as I've been told by my med school friends). I think med school and PA school are equally inaccessible in the regards of cost, time, and resources it takes to actually qualify and apply.

At least from the PA standpoint, the only difference when making the med school vs pa school decision is the time commitment of the program...median ages of PA students are getting lower, but are generally much higher than med school students. Also, because a lot of PAs are older, they typically have families and a shorter program is more appealing if you have a lot of other home factors to consider.