For riding through slush or light snow where there is little or no chance of ice or hardpacked snow underneath, narrow knobbies are great. I find narrow knobbies on heavily salted slushy roads to be able to cut through the slush and contact the bare pavement underneath. I have always had trouble with fatter tires (from 700 X 38s up to 26 X 2.2) because they seem to float and have nothing solid to grab onto.

If there is greater than 0.01% chance of ice or hardpacked snow then studs are best, any width. For riding in a city, narrower is probably better.