Sadly just test drove Audi S6 2013 and I am comapring it to my 550i 2012 With Dinan Stage 2...

Word is.... Man.. Good job Audi
Well I think I am very impressed with the handling more than power as With My Dinan S2 I believe power is about the same... though I felt that my 550i has longer breath and probably bigger lungs for some reason..

Exhauste: very nice with windows closed or open, loud enough but not intrusive by any means just the way I would want to to be vs my EV feel in my 550i.

Interior: well gauge cluster is definitely years ahead of BMW but rest of interior including console and dash 550i is a bit if not a lot more classier (I am 37 yo, so not old taste) ... Carbon Fiber inlays were nice ... Writing recognition is pretty darn accurate in a very impressive way!! Again good job Audi.

Handling: Audi S6 hands down by far, 550i does feel like on a cushion which I appreciate most of the time till I wanna do some spirited driving then the smile on face goes away, too soft and isolated while Audi S6 is just done right, though I felt as a daily commuter you may like BMW 550i more as it won't beat you up But my test drive S6 had 20 inch wheels and my 550i has 19's.

Engine compartment: BMW 550i engine looks a lot better and bigger I think.

Back up camera: Took forever to come up on Audi for whatever reason but still very sharp and high res with guiding lines.

Height: Audi S6 sits perfectly while 550i feels a bit higher and this is why I am considering lowering springs ACS on mine if I don't trade it in for this S6 -

Dash: Again gauge cluster is a lot better in Audi but I like everything else in my 550i more..

Wheels: 20's are really big and very nice looking and brake calipers look very nice.

Seating Position: comparable in both but I may like seating position in my 550i 2012 I can sit perfectly satisfied while in Audi I still felt something is lacking though I didn't know what and it wasn't height as I was really low! May be because I have the comfort seating package in my 550i compared to the diamond stitching sports seats in the S6.

Steering: feel is better and lighter and sharper than 550i but mine is the electric horrid 550i steering, BUT steering wheel in my 550i M is a lot thicker and beefier which gives it a better sporty feel even if it doesn't feel that way for all driving purposes -; )

In a nutshell
I like the Audi S6 more and I agree with all people placing it a bit higher than 550i and lower than M5... Midway and may be closer to the M than it is to the 550i.