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You'd have to drive both to make your decision, and I know you haven't done that since the Audi 5 series isn't out yet.
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I agree with Threxx that a test-drive is mandatory before a decision is made, and while I have not formally driven or reviewed either car yet (I HAVE reviewed the BMW 550i, the 650's brother), from what I do know about both cars, I'd say the Audi will probably will be a better choice. IMO it looks better, has a better interior, has AWD for better traction, and will probably be at least a little more reliable. There's almost no question that the 650 will have better steering gear, but IMO there is little else to recommend it over the upcoming S5.
I dislike both the BMW I-Drive and the Audi MMI controllers, however....that is one area where both cars will probably need improvement.