PRM is more difficult to pass than FRM. To get your PRM, you must pass 4 tests (2 if you are a CFA charterholder), whereas there is only 1 test for the FRM. FRM has a work experience requirement, whereas PRM does not.
FRM is probably more widely-recognized among employers because it has a longer history, however PRM is quickly gaining ground.
In terms of exam structure/ cost:
FRM: one paper, once a year, member fee
PRM: 4 papers, anytime, no member fee
So, if you want to learn something new in risk management and do not want to spend too much time on the things that you already know, I think PRM is a good choice.