Here in the Netherlands, we have a dealer who helps you choose the right one.
He just installs one, let you hear it, then installs the next until you tell him it's ok.
Got a stainless steel cherry bomb under my 2.5.
When idling or at "normal" pace; it is pretty quiet.
When pushing the throttle there is a deep dark rumble.
I like the sound alot.
Costs about €80,=