In the Tirerack survey results the BFG g-force T/A KDW gets great handling ratings, but rates pretty low for ride comfort, noise and wear. 18's on an S6 might be pretty harsh.
http://www.tirerack.com/tires/surveyresults/surveydisplay.jsp?type=UHP&width=235%2F&ratio=40&diameter=18&tireSearch=trueI don't have any experience with either of the tires you listed, but I love the Goodyear F1 GS-D3's that I have on my Corrado. I haven't tried them on my S6 because I run ultra high-performance all-season tires year round on that car. Avon Tech M550 A/S 234/45-17 to be exact. The Avon's are a great value, but I may try a set of Bridgestone Potenza RE960AS Pole Position next to see if the extra money is worth it.
The ultra high-performance all-season tires are a great balance of good grip in summer and decent snow draction in winter. We don't get more than a couple big snow storms around here that would require better gripping snow tires. Most of the time it's just occasional light snow with the roads quickly becoming wet. The majority of winter is not driven on snowy roads so I bias my tire selection toward good wet/dry performance with decent light now traction.
If I was to get a 2nd set of tires for summer I would go with another set of Goodyear F1's or Bridgestone Potenza RE050A Pole Position.
