Also consider your preference in "regulation". An unregulated light will just keep getting dimmer and dimmer until it finally goes out. You have plenty of advance warning that you need to replace the batteries. [ But many people seem to ignore that warning. For some unknown reason they prefer to shake and beat on the light in a futile ritual to get it to turn back on. Strange folks they are - shaking a dead battery doesn't make it spring back to life. You see this all the time in the movies. ]

Anyway, ...

With a highly regulated light, the brightness stays the same, or very close to it, throughout the battery life. The lights I carry daily are all highly regulated. To the (almost ridiculous) point of staying at full power and then just suddenly turning off when they're dead. Luckily, I can switch all my lights to a much lower power setting and turn them back on for a few more minutes to a few more hours after "lights out!", so I'm not totally in the dark. But it is a little disconcerting for the first few seconds after your blazingly bright flashlight just turns off instantaneously. Until you remember to turn it back on using a lower power level and go grab some other freshly charged batteries.