I wouldn't be worried about psychologists universally getting prescriptive rights any time soon. Currently, psychologists only have prescriptive rights in Louisiana and New Mexico and for some psychologists in the military. There are indeed some "turf wars" going on in the mental health field, but this particular battle is looking like a longer road. Unless there is some dramatic new development or shift in the next few years, psychologists face a long uphill battle on this issue. Tons of opposition from psychiatrists just to mention one resistant factor.
In terms of there being a difference between a PMHNP and a clinical psychologist if psychologists were to get rights on a broad scale:
There would be a big difference in that on top of their prescriptive rights, a clinical psychologist would have at least 4-6 years of supervised psychotherapy experience in a variety of settings (including an internship) as well as the ability to conduct and interpret psychological testing, not mention extensive research training.
Most Psych NP programs are 2-3 years and don't have the same depth of psychotherapy training.
However, right now, in my humble opinion, PMHNP is the best bang for your buck in terms of training time, scope of practice, and income potential if you are looking to get into mental health. As a PMHNP, you might not be a "doctor" but you can do therapy, manage meds, and have a private practice (varying scope by state) in a much shorter period of time with better earning potential right out of the gate.
Full disclosure, I write this as one who was originally planning to go into a clinical psychology PhD program and has now switched and is pursing PMHNP.