Jeff - I think you're refering to the old 4Matic as being "reactive". The new 4Matic is true full time AWD like Quattro. The biggest difference between Quattro and 4Matic is that the Quattro can vary the torque split between axles using the center differential while 4Matic uses traction control to accomplish much the same result.

In any event, if driving "hard/spirited in really bad conditions" or passing on a crowned road in the ice" I would much rather be in a Mercedes than an Audi for one simple reason. Sooner or later you are going to hit something real HARD.