First hand experience on my legacy.
195/65/15 - Studded snow
205/70/15 - Studded snow
Granted these tires weren't the same tire and brand, but both were in great shape. The larger tire was leaps and bounds better in basically every condition I encountered (which is every condition possible - serious, I drive in snow and ice almost every day). That being said, I think the reason it performed better is the overall diameter, not the extra width. However, I've been toying with the idea of trying a 215/70 or 65 to see what happens.
I do encounter snow conditions where I'd rather stay on top than sink and bite, the snow is heavy and thick. Once you start sinking it kills momentum and control, I think this is where having some more girth would help. This is also the type of snow I seem to get stuck in the most. There are two ways to attack this snow, lug it and try to stay on top and maintain momentum, or just spin the tires and use hp to pull you through. Many times I'd rather just lug it and try to stay on top, but I can feel the tires crush through. I probably get high centered and stuck 5-10 times a season, mostly in this type of snow.
Most of you probably never deal with this on a weekly basis. For general road use, go with like has been said, more diameter and close to the stock width.