The FMF pipes I have heard were really quiet. I also did not like the way it looked. I don't remember what series it was. I do know they make a good pipe both companys you asked about.
I like a good sounding pipe! Load or quiet does not matter to me. I have a stock exhaust on mine. It sounds like crap. I just ride it on the trails and the power is good. Stock 400ex
On my 450r I had a curtis sparks. It was load but I usually had it on the track so who cares everyones quad was load.
The fmf makes a great exhaust and I know its quiet. I would probly get that because I have heard it and it made power on the quad it was on.It was a 450r I would recomend FMF!!