Narrow equals more 'cutting' through to the pavement power vs. wide 'floating' on top of the snow.
I'd recommend running two sets of wheels and tires, summer and winter, so the salt doesn't trash the 'pretty' rims of summer.
We're in Germany and got tired of the 2 sets of wheels/tires for the wife's car (Focus) and went with Vredestein Quatrac 3 tires.
http://www.vredestein.com/Banden_Bandtypes.asp?UsersessionID=0&BandgroepID=3&BandtypeID=66
They are quiet, don't squirm and have outstanding grip both wet and dry for a tire that works very well in the snow. Highly recommended even if you use it as a dedicated 'winter' tire.
Had some Firestone Winterforces on winter rims for the 3-series a couple years ago and they were great in the snow but dangerous otherwise! Think brand new nobbie tires on your dirt bike and the looseness that comes with high speed. Anything more than a slow lane change caused the car to walk around on the knobs of these snow tires. They couldn't go around a corner to save their ass either...
So, my recommendation is to shop for a slightly narrower than stock snow tire that is multi-season capable since you travel mostly plowed roads.
Brian