You heard mine and it's really not that loud but everyone has their own preferences. Since you came from a mustang, you should be used to loud exhausts
Even if you get the 18" and think it's too quiet, you can always get some HFCs or test pipes to make it louder.
I haven't heard the 18" or SS in person so I can't compare but heard an HKS hi power recently and that is very quiet. I'm sure it's louder under load but just free revving it was very subdued.