My daily-driver S4 has S-Tronic and came fitted with a Milltek resonated system. For me the resonated system was way too quiet. It sounded only slightly louder than stock S4s I'd driven, but there was no drone in the cabin at all. I switched to non-res after a few months and never looked back. It now sounds like a proper V6, with a lovely tone and a proper bark when you nail it. The car is still quiet on idle and absolutely civilised when trundling around but that noise when you mash the throttle and on flat up-shifts is to die for. The only downside is that there is definite drone at motorway speeds in 7th gear, although dropping to 6th when climbing hills does help. It's not ideal when you have a car-full of passengers (no complaints yet but I'm aware that it's not the most comfortable of noises) but 99% of the time it's absolutely fine and there's no way I'd ever go back to non-res.
Interestingly though, regardless of res/non-res, the sound changes completely depending on the style of exhaust tips you have fitted - ovals acted like a trumpet and produce a lovely V6 worble, whereas the quad tips make more of a turbo-style roar. I much prefer the oval tip sound personally and therefore would be willing to sell the quads if anyone reading this fancies a change.
