Brightness or battery life , which matters more?
I don’t really care how bright my batteries are.
Do you mean run time?
For my EDC, I like a 1x21700 flashlight with a single high output emitter.
I use my EDC (which is a Convoy) on the 40% mode, which means that it doesn’t overheat, the output is reasonable, and the run time is somewhat decent.
So, I kinda choose medium/high brightness with medium run time. 
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No way to answer this question. It depends on the use case.
The professional answer is: you should only use the minimum lumens to get the job done. This strategy also maximizes battery life.
Now we are all guilty of using 10k lumens to go down a flight of stairs in lieu of turning on the ceiling light.
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Battery life hands down first, then power.
I will optimize the springs, the heat dissipation, the driver efficiency, the optical efficiency, the thermal interface material, interconnect size and LED efficiency to absolutely maximize runtime.
After everything is done, THEN we increase power.