Well....it depends, there are basically (and this is a rough generalisation) three different kinds of AWD systems:
1. constant, simple like Subaru and Audi Quattro: Subaru has traction split 50:50 between front and rear tires, always, no excpetions. Audi Quattro, if I remember correctly is the same, but 40:60.
2. Electronic, clever, self adjusting, like Mitsubishi or BMWs X-drive: the on-board computer constantly monitors traction on each wheel, your throttle position, speed etc. and distributes power, cuts off, adds baraking etc. however the situation demands.
3. "Fake" AWD systems: Systems that are normally FWD or heavily FWD biased and transfer traction to rear wheels only when the front ones start to slip. Volkswagen 4-motion to my understanding is one of these.
I myself prefer simple, mechanical systems, so my vote goes to Subaru and Audi. However the new smart and electronic systems are, according to many car magazines, pretty much better in every way and keep you out of trouble and you have to pretty much try to get yourself in the ditch with, say an x-drive BMW. I still prefer Subaru and Audi: you know exactly what it is and what it isn't, all the time.
On a track Mitsubishi and the likes are in the league of their own, because they give the power whereever and whenever it is needed, therefore making a much quicker lap time: in essence they are really, very fancy traction control and ESP systems, so you can (basically) just go through the corner full throttle and the computer will make sure you go as fast as possble. Nissan GT-R is perhaps the best example when talking about AWD machines going fast on a track. A conventional AWD car will understeer and the driver must counteract that behaviour.
I'm sure I have some inaccurasies here and there, so most likely pretty soon someone will come and give you more detailed info, because as I said this was only a rough way to divide different systems and each one is a bit different and it's not that simple when you really start picking apart the differences and characteristics of each system.