From my experiences in Montreal there are two factors to consider for snow tyres:
1. Compound - the softer the better, but be aware in mild climates they will wear very quickly and can over heat if driven out of season.
2. Sipes. The more the better. Ice is not slippery. It is the thin film of water on the surface that is. Sipes remove this water giving you a better chance of some grip.
As Will W said, 4x4' s are terrible on icy roads unless they are subarus or similar as the weight is a huge disadvantage - you may be able to pull away from the lights quicker than matey in his little astra, but you wont go around the corner or stop at the next lights quite so easily!
The Candians have a saying that goes something like " The first sign of winter is an SUV in a ditch!"
Your best bet for icy roads is a light front wheel drive car and gentle application of the controls. (Unless of course you have a Subaru). Our Golf with winter tyres was miles better than the Rangie on Trac edges on snowy roads.