I voted for the W12. It's just that in some cars, luxury matters more that sport. And it does in A8.
A8 was built to be a luxury sedan and not a fast one. I'm a luxury fun

and anytime i take luxury over sport.
Then since both have 450HP it's not difficult since you don't ''lose'' hp so it's up to you top decide.
And yes, the S8 grill is disgusting