Lisa, mmmmm let's get all those terms correct for ya. In the case of the BMW xi, under normal driving, rear wheels still have the most burden (not 100%) to drive the car forward. Only under extreme conditions will all wheels engage in driving the car. In the case of the Quattro in an Audi, all 4 wheels drive the car at the same time with equal bias under normal driving, and when it senses wheel spin it'll transfer power to the wheels with the most traction.
In the case of most SUV's, UNLESS they specify FULL TIME ALL WHEEL DRIVE, ONLY the front wheels drive the vehicle. These vehicles are often named or branded as 4WD. When the onboard computer senses wheel slip or after you press the button or work the lever that says 4WD, then all four wheels are engaged to work together. They are deceiving that in most systems you NEED to engage the lever or hit the button yourself to get all 4WD, and needless to say in emergency situations you won't have time to hit that button or lever. More deceiving is that, in these 4WD SUV's, it's recommended NOT to have all 4 wheels driving the vehicle at all times. It's recommended to use 4WD ONLY when you go to drive around in a mud pool or pulling your boat out of the water.
Now you see how useful the Quattro system and all other FULL TIME ALL WHEEL DRIVE vehicles are. With ESP in your Audi, you are very safe.
Now comparing the BMW xi and Audi Quattro cars. Read the Edmunds.com review on the BMW 325xi.
It's titled "This Bavarian Motor Doesn't Werk". The editor basically slams a huge rock at this BMW, and read what he recommends.
Bimmers come standard with Traction Control, but you can add this $500 option to your Audi. Looks like you live in Minneapolis (been there in the winter), and I wouldn't drive a bimmer that comes standard with rear wheel drive. Only the xi or the X5 will be on your list if you want a bimmer, even I'll cross the X5 off the list cuz it's an SUV and with high center of gravity it won't do well in curves.
One more selection to your list if the ES 300 is on yours. THe Acura 3.2TL Type S. 260hp for a very nice price. And it's a HOnda too.
Whew I typed too much again, didn't I.....Now for the reward for reading all these. For ANY OF YOU who lives close to Chicago, I got good news for ya. Buddy of mine told me yesterday that Audi is hosting a Quattro Challenge in the Chicago Motor Speedway. A full day of activies such as Cornering Control, Accidence Avoidance, Skid Recovery, Timed AutoCross, Speed Laps and Personal Coaching are all included with a dinner in the famed Field Museum, all for.....you guessed it, FREE. I can't believe it, and to qualify you only need to have a 97 and up Audi and you can bring a guest. But be fast, it's limited to 50 owners for each day on Saturday (9/8) and Sunday (9/9). Click on the following link: