I had a Subaru for the snow driving (now have a Ford Explorer) and it was fine in snow but sounds like you're describing ice. No vehicle can drive on ice, except maybe a zamboni with studded tires.

The Audi is four wheel drive (the Audi people try to make it special, call it "quattro" :-) and the Subaru is all wheel drive; there's a difference. Four wheel drive works 2 and 2 in pairs where as all wheel, each wheel works independently so the wheels with the best traction work (each wheel has a computer to do traction sensing).