The Audi has all wheel drive so it takes off a lot faster off the line and has better traction through the turns because four wheels are spinning instead of one.
The BMW however will catch up with the Audi on the straight-aways and will be a lot more fun to drive considering you can drift it.
Both cars have powerful engines but you would need to be more specific in asking "which one is faster?"
The BMW will be faster in a straight line, and the Audi wins in the curves.
But as far as looks the BMW ////M3 looks a lot better than the Audi RS4, and the M3 is easily distinguished from the other 3-series. But as for the RS4 the only seemingly difference is the badge on the front and rear.
Hope that Helps,
Jeff