The S3 AWD is similar to VW Golf system. The S4 is all Audi. I'm not sure how it comes out on the road, I guess most drivers wouldn't notice the difference. Before is was that the Golf AWD was Haldex-based, which meant the car was actually front-wheel driven most of the time, and the rear joined in when the wheel spin was detected. The A4/S4 system was AWD all the time. It seems this might not be the case anymore, since the S3 tehcnical data also says the all-wheel-drive system is constant.