I'd say it's down to temperature.
A winter boot at 20C in the wet will be worse than summer tyre in the wet at that temp... however remove 15C off that; at 5C and the winter tyre will become a lot better in the wet.
Winter boots start making sense around ~7C and below... in the slush/ice and snow there is simply no comparison the winter boot is far, far superior.
I think you will notice the biggest difference simply moving from summer to winter tyres in those conditions, the difference between brands you won't be able to tell because you cannot compare them.
I've personally used Dunlop 3D's, GY Ultragrips and this year I'll be trying the Continental TS 830P or possibly 850p if I can find them, on the fronts (still have plenty of tread on the GY's for the rear).
Makes winter driving much more pleasurable. What I find interesting about the 17 inch vs 18 inch winter tyres is the speed rating the 18's are V rated and the 17's H. But as long as your not doing over 130MPH in the snow, you should be ok.