Well...honestly, this is the tough situation a lot of people face. If you ride knobbies for snow, you're a lot slower on clear pavement. But if you ride skinnier studs you're completely safe for cleared (or relatively cleared) pavement, but it's tough when a snowstorm hits (though nothing works great then, frankly).

A lot of people have 2 winter bikes for this reason. Skinnier studded tires are definitely faster than the big knobby ones - the fastest I know of with excellent grip on ice is the Schwalbe Marathon Winter. I only lost about 1mph between those (35c) and the 28c summer tires that were on the bike before them. (Well - at full pressure when the worry is only occassional patches of ice, when there's a lot of ice I have to lower the pressure and they're not as fast, but nothing else is either).

On the other hand, a bigger knobbier tire is better in snow or in the "snow packed down on the road" kind of stuff.

So...I dunno. I know I live in Minnesota and personally I won't ride without studs in any conditions where there might be ice. I know people who do, but they always have a crash at least once a year no matter how big their non-studded tire is (well, not counting Pugsely riders, lol). So it's difficult for me to hold back from saying "Why are you running non-studded tires when there might be ice?" :-)