@MKNS to that I would say "cobblers". I have 700x25c Gatorskins on my normal everyday bike, 700x32c Continental Touring Plus (light tread) on my touring bike. I've just invested in some lightly spiked Schwalbe Winter 700x30c for the touring bike - why? Because even the wider, lightly treaded tyre offers absolutely nothing on ice but slidyness. Turn on ice and you'll go down sideways as the wheel slides our under you. Try brake on ice and you'll begin to slide and then likely go down as the tyre begins to slide on the ice.
The touring-type tyres were OK in proper, fresh snow I found winters before last, but when it came to compacted, refrozen slush, again the ride was very ropey. I agree with what the shop said about tread and no tread on ice - Bicycleworks said the same on tread vs. no tread for ice, and that the slick might even be better as it puts more rubber on the road than a knobble. A wide, semi-slick at low pressure probably the best non-specialist tyre for ice. Big footprint, some channels to clear away water of melting ice, and with a bit of sideways flexibility to counter the likelihood of wheelslide.
The studs however stop / slow lateral sliding, wheel lock when braking and wheel spin when accelerating or trying to get uphill.
There are a number of different stud / tread arrangements available. Chunky knobbles with lateral spikes are designed for off-road snow work. Central studs are suited to snowed / iced road/path work.
Having now ridden my studs for only 3 days, including one frost day and one snow morning, I know which I'd rather be using on untreated surfaces.