If that's the category of vehicle you are considering, I can't recommend the 750 x-drive enough. I did a 7 hour road trip with my FIL in his when he got it. Brilliant car.
The A8 is great, but I'd wait to see if they do a TDI model with AWD. The rumor of a TDI was being passed for the 2012 or 2013 model year.