Everyone has a opinion on this topic, do a search. My opinion, i had a F/M 40 and loved it, but when i installed the Magnaflow it was night and day in the performance category. The M/F out performed the F/M big time.
My Performance improved when i did the switch from the 04 pipes with the F/M to 07 pipes with the M/F. The 07 pipes do flow alot better than the 04-06 pipes. The best bang for the buck is the 07-08 exhaust with your choice of muffler, most guys are giving those pipes away. Just my .02.