I’ve used a Thule system with no problems whatsoever. Optional fairing adds sleekness and helps a little with gas mileage. The attachments are easy to change, and it holds solid. 2 fork mount bike trays for summer, and the ski/snowboard ones for winter. Though I don’t like driving around with the ski/snowboard ones on since they are more or less barn doors attached to your roof - terrible for gas mileage. This isn’t really a big deal since there are no tools for taking them on and off. The bike trays are much more aero and I leave them on even when not in use. The trays require tools to take on and off, but still not really an involved process. You can get locks for everything too (ski ones come with it) which is nice if you have to leave your car with equipment on unattended for a little while in an unfamiliar area.
You just need to get the cross-bars and mounts that fit you vehicle. Once you have cross-bars, you can purchase any add-ons that you want. I have a thule system and use a ski box and regular fork mount bike holder (just toss the front wheel in the trunk. If I want to run both the ski box and bike mount at the same time, I just need to move things around on the bars a little.
Not sure what the roof of the S6 looks like, but for my car I got the permanent roof rails attached by my rack shop. I prefer that method to the mounts that clamp onto your door rails. I can take my entire system on or off in about 5 min compared to the clamped mounts that need to be adjusted and tweaked if you take them on and off.
We have an A4 wagon, with a Yakima rack system for bikes (front fork locking, separate front wheel mounts) with a Mont Blanc pod for skiis/gear/etc. I XC ski and used to have a pass-thru in my Volvo ages ago so never needed roof rack (my small 49cm bike would fit in my trunk! ha!); the A4 doesn’t have a passthru, and my old A6 wagon (2002; between the Volvo and the A3 then the A4 wagon) didn’t either so the pod was what worked for us - long enough for my skis and good for other crap in the summer. The extra separate wheel holders (for the removed front wheels) are nice to have when we need to keep the trunk area clear(er). PM me if you want a pic.