If you have to ask, get lines. (In my opinion) there is no disadvantage to having lines, only advantage.
I wouldn't get a lined unless there was also some kind of coating (like a hard plastic or epoxy, etc) on the fingerboard to keep the inserts in (or if the lines are simply painted on). The real problem with most lined basses are they're simply factory "conversions" of the fretted models, with inserts of some kind put in the already-cut fret slots.
They eventually start to work their way out of the wood, especially if you get lots of variation in temp and humidity, causing the wood to shrink and expand. Then you have a real mess on your hands with these things popping up and causing buzzing everywhere......
Unlined eliminates this problem completely...
LS