General rule, the more traffic, the more comfortable it is to ride if there is a shoulder. Lots of traffic, two lane, no shoulder ... usually no fun at all. That said, it is always situation specific and selecting the best route depends on a lot of factors. A bad road surface can ruin an otherwise nice route and while you might get information on whether a road is paved or not, getting information on surface quality is much harder to come by and this is even more true for shoulders. Some unpaved roads are much nicer to ride on than a recently chip and sealed "paved" road.