Use the three inch if your after max performance, I would mount the muffs as far back as possible to allow the exhaust gases to cool and that will make it a tiny bit quieter also. My last two cars with three inch pipes had the muffs mounted behind the rear end between the 1/4 panels and the rear sub frames exiting with 45 degree turn downs at the rear bumper My next car will have 3.5 inch pipse all the way back with those muffs mounted the same way