4RM3D has summed it up pretty well. Basically, to get full value from PP, you need to have 6 units on the board. It is actually a very hard condition. You need a long round and plan very carefully and you can never be sure how long a round will go. With so much removal in the current meta, it is not guaranteed that you would get full value from PP. MA, to get the full usage is very easy and quick. Your third unit can tutor it go get full value.
But MA could give a very juicy Scorch or AC target with huge point cards on board for your opponent too. I haven't used both, but I would prefer MA over PP any day.