Lines if you'll be playing any kind of demanding music. Muscle memory works on an upright, but not so much on a BG. It's a whole different deal. Watch the great fretless BG players; they look at their hand while making shifts. And it almost doesn't matter if you can hear you messed up and correct quickly, because you just honked a clam. You can fake it by doing a vibrato and acting like it was on purpose, but that only works sometimes. Other times, just being a little out of tune for a split second makes the whole band sound bad. You don't get extra points for having no lines and/or not looking.
+1 and consistent with my experience both ways.
I guess there are two kinds of fretless bass players: those who like lines and those who don't.
One point is that everyone posting here is using some kind of visual reference; and IME that is required on EBG. Muscle memory is not happening, unless God has given you something no one else has. Side dots work OK.
I have no problem seeing my fingerboard at a glance, but that capability will depend on where the player holds the bass. I don't hold mine like a dobro either. the neck pup is about where my navel is. If you sling the bass down at your knees, then, yes the fretboard is hard to see.
Spade, you've posted this before, but I never get your point that the note isn't where the line is. Stopping the string directly on top of the line works on my instruments within micro-adjustment. Sure, if you are going to finger behind the fret line, like on a fretted instrument, your stop is in the wrong place. But, the line is much closer to the correct witness stop than side dots are going to get you, again IME.
Anyhow, these are personal prefs, and I doubt the differences will cease to exist. The main thing is to play well and in tune with taste. God bless us all to get there however each does it.