Quick Summary: Lux or Candela shouldn't make any difference. Except the vendor does it wrong :- )
Lux vs Candela? Ha! It's not about two but at least about
three:
- Lux - the illumination at the target, the spot.
Always keep in mind questioning: taken at 1m? (OK, that's the usual case), or another distance (then you'd have to recalculate)
- Candela - the luminous intensity of the source in a particular direction - that's the proper unit.
Usually it's easy: The number for Candela is the same as if you had measured Lux at the distance of 1m (~3.3ft). And for any other distance, you would just recalculate.
- Candlepowers - a historical unit. It's quite similar, but not exactly.
Why any difference to Candela at all? Before the Candela was defined, countries had different ways to define a "normal", using real lamps with specific dimensions, fuel etc. Example links: US, France, Germany, and so on.
After all, the issue was settled with a common, reasonable and scientific definition, the Candela.
The historic Candlepower is equal to 0.981 Candela. That slight difference wouldn't be a problem for us flashies. But guess what happened then: Crazy fantasy specifications for some US warehouse handheld lights. And then, some chinese chimed in and also tried their best about notoriously exaggerating some values...
Why Lux at all? (instead of Candela? Strictly speaking, Lux is just not the correct unit, anyway.)
Lux might have been in use for a long time, because it's just closer to what you do in
practice: There is no "candela meters", only "Lux meters". You take your luxmeter and read "Lux". But I hope you also have taken the distance to the source. Becaue Lux is all what your device can tell (without having a range finder built in :- ). Thus you either measure Lux at 1m, or you recalculate after measuring the actual distance. And often you even shouldn't use the close distance of 1m: Especially big throwers and/or lights with a high lens diameter only should be measured at much higher distance, where the beam profile is becoming uniform...
I like the term candela because it gives you the info at the same distance every time. It helps me understand whether a beam is like a pencil or a flood light or in between.
I have a pair of Pelican 2350's for example. The 100 lumen version has 19k candela. The 175 lumen version has 9k candela. Now obviously the 175 lumen one is "brighter".
I guess if the lights have
similar Lumens, then comparing Candelas can work, thinking about "pencil" or "flood".
But keep in mind:
strictly speaking, Candela or Lux really doesn't tell
anything about "pencil" or "flood". Without the Lumens information, this only tells about one very direction of your beam. It doesn't explain whether you only have a
tiny laser-like pencil beam with some spot-Lux, or some enormous flooder enlightening the
whole field with these Lux (what a difference...)