The PM is the better of the 2 units, built better, and I have seen many threads with people having probs with the K2R, just remember that you will pay more for quality. You can get a K2R for less than $250 with media, the PM unit alone is over $300, but you get what you pay for.
I was using a K2R. It seemed to work fine. I think the best advice is to use your return pump to feed your reactor. If you use a powerhead you will get some inconsistent results.
Now I am using a dual stage Geo. It costs less than 1/2 of the K2R. Same results. I have only been running it for 2 weeks though.