I have actually seen a couple of new 2005 A6's while test driving an S8 at the dealer. A few reasons a new A6 4.2 might be desirable over the old S8:
1) Nearly the same horsepower ( 330 vs.. 360 ) and less weight. Also has chain driven camshafts rather than timing belts. Eliminates or minimizes expensive maintenance.
2) Vastly improved navigation system, integrated into the MMI in the center dash. A nice 7" color screen, and DVD-based rather than CD-based. The old S8 nav system is a PITA to use, and it has no screen at all, just arrows in the auto-check window.
3) And, of course, less mileage! Longer warranty period ( 4yr/50K miles ). The A6 impresses me as a larger, more luxurious, and better equipped car than its predecessor, and it even has a larger trunk than the S8. I asked the Porsche sales manager and the Audi sales manager why anyone would buy the A8, and they were speechless! The interior design is far superior to the old A6 or A8/S8. Sport seats are included that are very similar to the S8 sport seats. The CD changer is in the glove compartment rather than the trunk, and the integrated phone is detachable from the armrest to function as a portable cell phone.
Gee! I guess I've sold myself.....