The other point I would offer than is that BMW x-drive is a RWD-based system, while Audi's are FWD biased. BMW puts all the power to the rear and diverts to the front on slippage. It's quite a thing, especially when you put it in DTC mode, you can drive it like an old American car in the snow, powering/drifting through the turns vs. pulling/compensating for where you wanted to go. If only you could test drive both in a snowstorm (and know to press the DTC button) you'd be sold.
again... good luck with your purchase decision. Audi is fine too.
07 328xi coupe SG/Dakota leather, 6-speed manual, sport/premium/cold
02 325Cic vert AW/Montana leather, 5-speed manual, premium/removable hardtop
93 850Ci Oxford Green/Parchment, auto
71 Buick Riviera, 455 V8 land yacht