Back roads unless they're totally impassable. Main roads are just a barrage of aggressiveness here and the volume of traffic makes it that much unsafer, i don't mind being passed occasionally but when you get 50-100 cars squeezing you in between a snowbank and the tire-tracks it just gets nuts and really increases your chances of being hit. The one caveat would be if this main road has exceptionally wide lanes.

Anyways, when you encounter people who honk at you and are too chicken-**** to pass or are driving with bald tires and will instantly lose control when they leave the tire-tracks... slow down and move to the right then stop if necessary. Winter cycling isn't a race so stopping for 2 seconds to let a motorist by who is making trouble is well worth it.

Whichever you pick be safe and enjoy doing it.