Remember with Supersprint extra cost that you will have to budget for a new rear diffuser to be cut and plastic welded to accommodate the extra tailpipes on the offside.
I don't think any of the performance offerings will be a lot louder as the tendency is to keep the car discrete when it's going round town, then when you get some open road and give it some they will open their lungs.
If money was no object I'd have Supersprint, but I have an MV Sport / Blueflame sat in waiting to go on and I was more than happy with what that delivered sound wise (100 cell non resonated).
I wouldn't fit a Milltek to the S3 without doing the rear engine mount as there have been some issues with DP's cracking and the mounting brackets giving in. I think the rear mount moves too much which is no fault of Millteks, but after having one on my previous car I really can't see why everyone holds them in such elevated esteem, they're nice but they're NOT exceptional. At the end of the day they're all just high quality drain pipes!