For the nastiest, worst conditonsTotally agree with above post. Studs offer the best traction for the worst slippery conditions - black ice, ice storms with freezing rain, new snowfall where it's around 32 and super slick etc.
Here in the NW the conditions are closer to freezing and it can get really slick, melting-freezing cycles build up the black ice, studs really pay off. Much of the time in the rockies it's so cold the snow is sticky, but you just need to decide if you want put up with the noise and marginally worse dry-road traction of studs.
I've always went with a seperate set of studded tires except for my newest car - studded or even studless snowtires weren't available in the correct size until this year and I'll probably pick up a set for next winter. Worst case so far this year I have just used the 4x4 with the studded tires and left the new car at home.