Originally Posted by racingbmwm3

1. It is UNSAFE to mix winter tires with all seasons. Do yourself a favor and get a second set of wheels with winter tires. As soon as you know you won't be making anymore snow trips, put your summers back on the car and then replace the front tires with another pair of non-winter tires.
I recommend the 255/55/18 for winter tire size. Much cheaper than 20" tires, and for winter traction, skinnier tires are better than wide, and this is a common size. You don't mention what model X5 you have, so you may even have an option for 17" wheels/tires which would be additional savings in overall cost...what model do you have?

2. Additionally beyond the excessive wear in summer, winter tires offer way less grip on warm dry roads, so your X won't handle or stop as well as it would with the all seasons.