In another publication the Xdrive does even better than the quattro, see here.
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The quattro seems to struggle quite a bit with just one driven wheel..
They need to start poking on and off all manner of systems to make it move.. Not really that impressive.
How much of this translates to real world driving is hard for me to judge. I know that Audis generally project more "safety" and calmness on wintry roads, because they have like 60% of the weight over (or in front of) the front axle, while the BMWs can feel loose over the rears, but when the Audi starts loosing grip it just ploughs in the direction where it came from. It's understeer, understeer understeer..