How much life are you going to get from your batteries?
That may not be the biggest question you are going to ask yourself when you are looking into carrying a flashlight, but as someone who has seen what happens when an Eotech combat gun sight goes dead and the batteries need to be changed, there are times that you may be in a situation and the batteries are too low to be of use or your flashlight has just gone completely dead. The great think about LED technology is that that the flashlights will fade in strength and not just burn out all of a sudden at an unknown time.
The Surefire G2 is one of the most popular flashlights i usually see on M4 Carbines, but another company got smart and went with the Polytac cases for strength and durability. Streamlight flashlights are my primary flashlight even though I know there are manufacturers in the same price range that are pretty good, there are concealed carry or defensive flashlights with a Strobe, and there are flashlights with the strobe that you can use on a handgun or rifle in need be. It is another tactical worth considering.