I think they've made some subtle changes in the 4Matic torque split around 2007 from 40/60 to 45/55 but that's about it. I don't have much real life experience with either system so I'll defer to you.... but I'll be getting a lot of time with my new to me 4matic real soon.

FWIW, I have a BMW X5 and I found that changing the tires from the stock Michilen MXVs to Goodyear Triple Treads made more difference than I would ever imagine. Much more than any subtle difference on how any AWD responds to slippage. With the Michilens the X5 was a cow on ice, with the Goodyears, I'd take on any Audi any time on ice or hard pack snow. I know the Goodyears are plain old all season tires worthy of a minivan but frankly, they are the most impressive bad weather tire I've ever driven, even including dedicated Blizaaks.

As far as the OP's question of Buy now or buy later.... Well, I would put off any major purchase right now. Buy later, while bad for the nation, is good for the pocket book. Used car prices will continue to CRASH and more and more really sweet late model Mercedes' will be flooding the market as they come off lease. I bought an '02 in August, I think I would have saved 20% if I held out until now. My guess is that you would save at least 10% more if you wait until deep winter.