Tire size doesn't matter in snow, but the 18" rims have real tire choices for winter tires, while the 20" tires only have a couple of winter tire choices.
Tire compound makes the difference in snow. (ei: winter tires)
The 18's leave more sidewall in the tire, effectively helping ride quality, and reducing handling. And on stock trims, come with a meduim dampened suspension setup, and ok steering feel.
The 20's have less sidewall in the tire, effectively improving handling, with a small penalty to ride quality. (something to think about depending on your road surfaces) On stock trims, come with a dual mode suspension setup and excellent steering feel.