S8 on the left, 2.8QS on the right. I get 24mpg from both of them, although I do drive the 2.8 in S mode everywhere because the gearbox programming is so stupid.
As Ian said, the FWD is better on fuel and friskier as its carrying less weight and has less drivetrain loss. Early facelift (99-00) come in 2.8 FWD sport, but I think by 2001 everything was quattro.
Fuel aside, the 2.8 will have
slightly reduced maintenance cost as the engine is more common than the V8 so parts like the timing belt kit, water pump, aux belt drive are cheaper. Oh, and it has 2 fewer spark plugs. Thats really it though - everything else on the car is the same as the V8.
I have been contemplating selling the 2.8 recently. I love it to bits, but the gearbox makes me cross every time I drive it, which is not often enough to justify keeping it. Offers would need to start with a 5 though, which would buy a reasonable S8...
Thread about the car here:
Big Red What's in the Workshop
More pics here
Tornado Red 2.8QS