My own considerations are:
In Denmark we do get snow but come past midday the roads have been cleared so no need for wintertyres that are slim as i don't need to drive through inches of snow on the main roads.
At the same time i'm no fan of going down in rimsize as you lose the responsiveness of the steering by doing so. So the last 2 years has been with 225/45-17 and now i'll try with 18".
My personal experience is that when the sun can't dry the roadsurface before midday you are better of with wintertyres. The temp of the surface is to cold for it to help the sun dry the dew off.