My friend and I each had the FMF on our KDX250's; don't know of anyone that has used the PC.
The FMF is a good mod for the KDX250. It saves a lot of weight, doesn't add much sound, smooths out the low/midrange hit and gives quite a few more revs on top w/out taking away from the strong bottom end.
I have the PC pipe and silencer and I really like it. Ever since i put it one it was an instant gain on top and even gained a little on the bottom. The only downside is the noise factor which is a little louder than fmf but it is worth it for the power gain.
I have used both pipes on my bike, and I prefer the FMF, but it is really a personal preference. The FMF has a little more meat in the midrange, but the PC pipe revs a little higher, and is a little smoother through the middle. The differences are not dramatic, and either pipe is a huge improvement over the stock boat anchor that Kawasaki calls a pipe. I did think the FMF was a little easier to jet with, it seems to run a little cleaner down low.