TheGr8Swami, I owned 4 Audi before my current BMW and I find both dealership's customer service very good, in fact, I thought the Audi's customer service was a bit better. I had a lemon buyback with Audi and had no problem what so ever. I guess it really comes down to the individual dealerships.
Thrumcap, looking at the numbers, the 550i shoould be able to keep up with the S6. Handling wise, I think the S6 has the edge.