The non-studded vs. studded winter tyre debate has been going on for ages on car tyres. When they test the various tyres, the result is always the same there: on ice and hard packed snow, nothing works like a studded tyre. There are big differences in dynamics of bike tyre vs. car tyre (think hydroplaning for example), but on this point I think the result is the same, for rather obvious reasons.

In my mind the question of studded tyres comes down to whether or not you have ice or hard packed snow where you ride. If not, you're probably better off with aggressive knobby tyre with no studs.

--J