My 2006 BMW 530i was totaled in an accident after 3 months and since it took 6 weeks for the repair shop to get BMW to determine it was totaled, I elected to go with an A6. Both are great cars. The 530 has better low end torque and is quicker off the line. Both have a great road feel. The Audi Navigation and computer are much more user friendly. As far as gas mileage, I was getting 20+ with the BMW. I'm getting similar mileage with the Audi although my last tank was around 18 - 19.
Great looks, more user friendly computer, excellent handling and road feel