What I have read from reviews in Europe there is also a difference in a negative way with the current version of x-drive in F10 (more weight, inconsistent behaviour, understeer/oversteer etc). According to reviews Audi has handled their AWD system in A6 better than BMW in the 5-series (yes it is too bad, Munich has to do something...
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If I did not need AWD and money was no issue I would definitely choose the M5. It is the perfect luxury automobile with sports car abilities!
Good luck with your choice!