Jen, I just picked up a set of Sottozero/ BMW 307 rims package from a local dealership. They were last year's stock so I saved $1100 over the current price of $3700.
To be honest I would have preferred Blizzaks because of our previous experiences with that tire and its excellent winter grip.
From online reviews, the Sottozero should be a better cold weather tire both in wet & dry conditions while not being as good as the Blizzaks in the deep snow. The all wheel drive should compensate for the deep snow conditions though.
Here is a youtube video showing the Sottozero's on an X3: