As a former Audi owner and fan of the marque I'd say go for the A8. At $35K with 53000 miles sounds like an excellent deal. Besides the car is really state of the art, aluminum body, gorgeous interior, quattro all wheel drive, etc... One important thing, at 53000 miles the free maintenance deal is over but the warranty should go for 5 years 100k miles I think.See if you can have the dealer throw in some kind of aditional warranty. Audi's are excellent cars but parts are incredibly expensive, much more than MB so buyer beware. Never use a shop other than an authorized dealership for service and repairs. My 1985 5000S had 180000 miles on it when I sold it and I never had any major problems with it because it was dealer maintained all its life.
Good luck.
Joe Brasileiro
1980 450SEL
1987 Jaguar XJ6