I've got an FMF Q4 with the FMF head pipe.
It seems that the FMF is more popular, maybe because they were available for the WR-250R and X before the RS2.
Myself, I think the FMF mufflers look better, and I know the Q4 ain't too loud.
I had to run my WR-250R without a fuel programmer for one ride after my Power Commander quit working.
I had the FMF exhaust on the bike, and I don't remember the popping being too bad.
What I do feel is this:
The bike runs and responds to the throttle so much better with a fuel programmer that I'd probably get that before the exhaust if money were tight. :)