Interesting topic. I recently cross shopped the two cars, and went with the 550ix. An S6 optioned equivalently stickered for slightly over 87 grand. My 550ix was 76 grand but I was able to negotiate close to 12 grand off sticker (end of year remaining 2012 model). I added the Dinan stage 2 software for about 3 grand and have never looked back. On my S6 test drive (which was exceedingly difficult to wrangle) I was able to dig in a bit, obviously no LC was used, and to my butt dyno the Dinan stage 2 and S6 feel very similar to me (perhaps a tad more hesitation in certain rpm ranges from the BMW). I would wager at the track the 1/4 mi times would be very similar, and on the street almost identical. I have a feeling the launch control would be used (at least by me, very seldom). I simply couldn't justify spending 10k more for the audi. If there was no wiggle room on the BMW, then I would have gotten the Audi. Both cars are wonderful, but unfortunately fall short of my 997.1 911 Turbo I sold. On the upside they both can fit my family of five.
2012 Dinan stage 2 BMW 550ix, 2007 911 Turbo, 2005 997 911 C2S, 07 BMW Z4M Coupe, 07 BMW 335xi