Who makes more money; a physician assistant or nurse practitioner without their own office?

I have heard from a good source, who is also a MSN nurse practitioner that P.A.s with several years of experience, similiar in duration to a NP, make more. I don't know why that is because they do essentially the same think. But I am guessing that P.A.s are sometimes given positions that offer a bit more autonomy. Or maybe because there are more male P.A.s--not that they deserve more because of gender, but that is sometimes a factor nonetheless.