I've shot both, I would go with the PMR myself out of preference, however the Mini does come with a better trigger and feedneck out of the box.
The PMR has a good upgrade path if you are that way inclined, the Mini is limited in that respect but acceptable as-is.
The Mini is extremely small and light, and fairly well balanced. The PMR is taller, longer and marginally heavier (it is still very small, and only a few ounces heavier than the Mini) and is also fairly well balanced.
The Mini's i've seen so far have been a little harsh on paint, the PMR seems to be a little easier on the balls.
There have also been some horror stories about the reliability of th Mini, check out the Mini section on PBnation.
The PMRs have excellent Tech support via Dye dealers (eg Heaven Paintball) and Dye themselves, whereas it could be argued that NPS could be a bit sketchy in that respect (Spares availability etc)
The best bet would be to try both guns out and see how they shoot. Make the choice from there. I prefer the PMR because it shoots very much like any other DM/PM, the upgrade path, and the easy access to Tech support.