Both cars are German.
Both cars are full-size cars.
Both cars are luxury cars.
Both cars are above $70,000.
Both cars have over 300 horsepower.
Both cars have under 400 horsepower.
Both cars take 5.7 seconds to get to 60mph from a dead stop.
Both cars have a standard navigation system w/voice recognition, air conditioning w/dual-zone climate controls, leather upholstery, memory system, 16-way power front seats, heated front seats, wireless cell phone link, universal garage door opener, power sunroof,
Only the Audi A8 has standard heated rear seats, trunk-lid closing assist, Bose radio, satellite radio, 6-disc CD changer, outside-temperature indicator,
Only the BMW 750i has a standard cooled center console and assist system.
The BMW 750i has 10 more horsepower.
The BMW 750i has 0.6 more liters.

The Audi is a little more luxurious, though the BMW is a little sportier. I'd rather have the Audi. What about you?

I can give a better opinion on this one, as I have driven both cars (about 3 years ago, A8 L vs 745Li). I thought I would hands down prefer the Audi, and I was biased towards it going into the test drive. But after having driven both cars, the BMW was clearly a more responsive and better driving car. It seemed faster, handled better and even had a smoother ride. And there was no comparison for steering feel. The Audi felt underpowered in comparison (330hp then). I much prefered the look of the Audi at the time, and the interior was far more pleasing (I never really liked the interior of the outgoing 7 series). That having been said, I'd be interested to drive both cars again, now that the Audi has had some minor tweaks and the 7 series has been re-designed.