my personal opinion is that the BMW AWD is better. I have had a chance to drive the quattro audi and my 325xi, and I really feel that mibne was a lot more solid in the snow. In the rain, the BMW wins, hands down. The sudi just feels strange when i druve it, and it feels uneasy in the snow.
both cars are great, though; the BMW has an advantage in my opinion in the AWD, but the advantage could just be my personal preferance......