I got a used PM9 a while back and liked it so much I wanted another. This time around I found a great deal on a new CM9. If I hadn't found the good deal originally on the PM9 I would have gone with the CM9 and probably been very happy with it, as I am with the new one I just got.
There is a slight dimensional difference in the slide due to the different machining which was mentioned- the CM9 is just slightly wider across the top overall.
The front sight is also plastic which I don't believe was mentioned. I don't see this as a big drawback and do plan on getting the front Mepro sight sooner or later. I did put a set of Mepro's on the PM9.
All that being said, I don't find enough of a difference to be significant.