I have owned both cars. You don't say what gearboxes the cars have, the step-tronic auto BMW box is quite slow to change, but the later style DCT box is as fast as an S-Tronic, so this would give the BMW a huge advantage if fitted.
The BMW would struggle for initial traction, but once rolling it will squat down and head for the horizon! I think the Audi would have it off the line, but the BMW would slowly start to claw it back. It would be very close at the finish!