Dry pavement: slicks are best.
Wet pavement: still slicks
Ice on pavement: studs
Snow on pavement: knobbies = studs (since most studded tires = knobbies)
Snow on ice on pavement: studs.

Me? Winter = studs, but 90% of the time I don't need them. The problem, is that the other 10% is not predictable.