I once owned an A4 w/ Quattro. I compared it very thouroughly with my friends IS250 AWd when i had the Audi (which i have since sold and bought a lex)....and, besides the systems being mechanically different (lexus as well as most other AWD systems are Haldex based) in pretty much every real world test Quattro is the champion, hands down. As a matter of fact, if you like i can send you some links of professional comparison tests that pit Quattro vs. various other car makers AWD systems. I live in seattle, so i dont need such a bad !Removed! awd system like the Quattro, thats why i know have a lexus (purchased for various other reasons than the all wheel drive).
I would never hesitate to go up to the mountains in any conditions in the winter w/ my old Audi A4, however, now, I take my girlfriends CRV because in my personal experience it performs much better than the lexus which i took up twice and it has taught me a lesson to never take it up again. However, for mild inclement weather and rain it performes beautifully.