Distance, for the same reason some others have put forth: Ultimately I need to end up back at home, and stopping at some arbitrary location presents logistical inconveniences.
I generally set off on a known-distance loop. I also have a pretty good idea of the pace I’ll run, so time and distance are strongly correlated - as kenobi 65 said, the distance I choose is based on several factors including available time.
I could pretty easily do an out-and-back with the turnaround at some particular time (half of my goal time if I want to assume constant speed), but I don’t often do that out of what could be called laziness: I like to know how fast I was going, which requires knowledge of the distance. I measure my runs’ distances by the (relatively) old-school technique of Google Maps on a PC, which isn’t terribly hard but does require a few minutes of effort. I find it easier just to do one of a my already-“measured” loops than messing around online to get the distance of my just-completed run every time.