Originally Posted by footie
I don't want to say something out of place here, just in case you happen to drive a 335ix but I will tell you this, when you start to compare the x-drive to quattro there isn't any comparison.
Quattro has an uncanny nack at finding grip where there is none, the brother drives an X5 and where his car starts to spin the wheels due to very slippery conditions my S5 has no problems at all.
I am sure many here will disagree with this and show numerous evidence for it, I can only say what I personally have experienced.
I hate to get involved in this one, but just as Quattro does not refer to any specific AWD system, and the different systems Audi offers vary in quality, the same is true for BMW. I am guessing that your brother has the last gen X5?
BMW has made substantial improvement to their AWD system in recent years. The AWD in my 2003 325xi was laughable, I have heard numerous reports though that the newer system is substantially better, but I have no personal experience with it. I have also owned a 2004 S4 and although the system was far better than the 2003 325xi it is still not as good as my Subaru 2.5GT. Ultimatley it's all relative.
In terms of the 335xi vs. S5 AWD, I would be leery of declaring a victor without seeing an extensive test or conducting extensive testing of my own.