I live in Colorado and drive a '98 3 series coupe (RWD). Like others have said, as long as you get a set of good snow tires (Blizzaks are usually a solid choice), you'll be fine. I've had my car for about six years and I've never felt like I needed or really wanted AWD. The only cases where it's really useful in CO are if you have a steep driveway at your house or if you go into the mountains frequently during the winter.
That's my opinion anyway. Most people will buy xDrive models here of course, but for me advantages are small and the downsides are considerable (the car is more expensive, slightly worse gas mileage, the car isn't as balanced as a normal RWD BMW). Do what you want though. If you'd feel better with an xDrive and don't mind the cost, then go for it. Snow tires are still useful though (AWD doesn't help you stop any better).