What I want to see is a new shootout by one of the various magazines.
550xi vs S6 vs E550 4Matic Sport
all top spec, loaded up with all the various driving options/adaptive suspensions, etc. Would be interesting to see. The Mercedes is probably the more luxurious, S6 being the fastest, and the F10 is a mix of the two.
Having driven most of these cars, stock versus stock, the A6 3.0T is dynamically a better car than the 535xi. Same with 550xi vs S6. BUT when you look at ride comfort, luxury, BMW wins out
The Audi and BMW have sort of switched their roles with BMW being the more luxurious one, whereas Audi is the sportier one. Both are fantastic. You can't go wrong with either. If we base it entirely on pricing/dealer experience, even I would say the BMW would win because the dealers are better and you can get a fantastic deal on a 5. Can't be done on a A6/S6.
That being said, I'm just waiting for the diesel variants of both before I make my decision. The MB is out because no AWD. Leaves me with A6/7 TDI vs 535d