I recommend going with a winter wheel/tire setup, even if it's an all-season tire. The wide 20"ers look great, but IMO I don't think are going to be effective in snow.

You could go for a M+S setup on 18s if you wish. I put a set of "High performance snow tires" on the X3 over the winter because we don't frequently get tons of snow and do plenty of dry road driving in the winter, but there's those occasional times where you do want the benefits a snow tire offers. Hence, I chose the Pirelli 210 Snowsports.

*Offers 95% of the dry road, highway performance/quietness of our M+S
*It's about 70% better in the slick stuff than our M+S
*The only tradeoff I've noticed is more tire squeal when you push it to dry road handling limits, and maybe a hair less responsive in the twisties.

Not a bad deal at all. Plus, they're H-rated and look much better than many of the hardcore winter setups.