I agree with Bobbyv on the role of limited differential.
I'd also add that though MB's 4-matic may handle some extreme conditions that most 4WD systems can not handle, like car is still mobile even only one wheel has traction, Audi's quattro 4WD still handles better than 4-Matic.
The problem with 4-matic is that it can't prevent slippage, it detects slippage then reacts. Audi's pure mechanical torsion differential is sensitive to the torque, it can transfer torque before slippage happens, I still feel Quattro is the best performing 4WD, the drawback is that its extra weight and fuel consumption.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.