I've owned my GTI since 2008 and have just got round to replacing the standard system by fitting a non-resonated (does away with the intermediate silencer) Milltek cat back system that I bought second hand last Summer - delay due to polishing it back up to as new then deciding to run the standard exhaust through another winter.
I did a lot of trawling of reviews and videos to help decide between a resonated and non-resonated system and with the knowledge that a turbo robs a lot of the natural exhaust note, took the plunge with the non-resonated version as like you I wanted a bit more sound without it being over the top for daily duties.
I fitted the system on the drive using ramps and a trolley jack to give some more clearance under the side of the car, and fitting was pleasingly straight forward as I renewed the clamps throughout with replacement Milltek ones. The quality of the system for second hand money is excellent and the tailpipes fill the bumper cut out perfectly and look like it should have been like that from the factory.
Once it was on, and with some trepidation, I started the car and the exhaust was... almost totally stock sounding at start up/idle. Driving delivers no intrusive noise on part throttle or when cruising at motorway speeds and there is only a bit more bite with a deeper note when pressing on. Absolutely perfect for keeping the peace with neighbours and the family on journeys. Therefore I don't think you wouldn't be disappointed with the non-resonated Milltek - particularly I you can find one second hand...
For comparison, I also have experience of two other cars with Milltek systems fitted (both naturally aspirated) as follows.
I offered up my Dad's Mk5 R32 to be used as the Milltek development car when they re-engineered the R32 system a few years back and that has plenty more sound due to the absence of a turbo, even with the resonated system in place. In DSG sport mode it sounds like a much more expensive sports car approaching, something like an Aston Martin, and you can hear the exhaust inside the car when cruising as well as pressing on. He loves it (oldest boy-racer in the area!) but I definitely wouldn't have wanted the non-resonated version on that as a naturally aspirated car (Milltek gave us the choice).
My brother has a Mk4 V6 Motion which I fitted a second hand non-resonated Milltek to last year, and whilst quieter than my Dad's car, certainly has more bite with the system fitted and sounds great - he loves his too. Again, even as an 'older' design, the quality is fantastic and it came up great with a polish in the garage before fitting.
As you can see from the other replies, there are many other options to choose from and whilst I can't comment on any of them from previous experience, hopefully my experiences with Milltek will help.