'I have heard both the resonated and the non resonated versions of the Milltek "in person" and I left scratching my head. Both do not justify any reason in my opinion to upgrade your exhaust. Barely tell the difference over stock. I am still debating on the GMG or AWE with or without the downpipes. I can tell you if it doesn't have an audible difference its going on ebay!!
Here's the difference in sound VERY CLEARLY on my own car before and after fitting, recorded with the same equipment on the same day with all the same settings:
BTW... Milltek do not do a non-resonated version for the S5 yet, so I can't see how you can comment on that one.
Finally, to answer the original question:
The Milltek does make the cabin noise more noticeable, which is great! When driven with the windows down it's just awesome! However whilst it is quiet-ish when cruising it's not silent. Below 2K revs it does 'boom' too. Unfortunately you can't have it all!