20' trench vs 100' trench would be my determining factor. 100' would probably be cheaper to run conduit. 20' would not be so bad to do it over again(not that conduits can/will break underground). If I was going under a driveway I'd run conduit and 2 spares. If I was tucking along a foundation for 20' and then digging over I could see myself tempted to use some of that 200' roll of UL cable I've had for 10 years and never used, but every time I get the trench dug I just end up using conduit.