I tend to prefer small, lightly trafficed roads. When I'm touring, I normally have some destination in mind. Roads usually go someplace. I've seen too many bike trails that stop and make me hike-a-bike back to a usable route (read: road), or dead end in the worst of all possible traffic situations, or go from sublime to mud puddles within a few miles (C&O, thinking of you!).
In urban areas, a bike trail too often is also a walk-five-people-abreast trail, or take-the-family-for-a-bike-ride trail, or walk-the-dog-on-a-25-foot-lead trail. Not to say there's anything wrong with those, I've enjoyed most of those activities myself. But when I'm touring, I'm not interested in doing loaded track stands to deal with them.