I have two separate weapon's lights for my AR-15.  One is a Surefire 65 lumen setup and the other is a Surefire 105 lumen set-up.  I work in a rural county with several small towns throughout the area.  The terrain goes between rolling farm land to mountainous timber land.  My question is this: are there any hard fast considerations to choose which is the best light for all around urban and rural areas.  Should there be a minimum lumen number for both.  I don't relish the thought of carrying a spotlight on my weapon to have the appropriate lighting.  Several guys in my department have 65 lumen setups, but I wonder about reaching out into the farm land and timber.  If this topic has been covered could some please direct me to the link.

Lumens are not an indication of range of performance.  If you dont want the weight or need the range of the 9V Surefires then The best compromise for size and performance in the 6V class is the Gladius. Gladius is good to about 50 yards. Standard 9V good to about 70 for comparison. 6V Surefires have less throw than the Gladius by a good 10 yards or so.

The Turboheadstyle lights will give the most throw.  The larger reflector focuses more of the light into a tighter beam that has less side spill.  A 105 lumen Surefire with the latest Turbohead will easily reach over a hundred yards.

From what I have gathered over time listening to experts and learning from experience.  60-65 lumen is minimum tactical application.  For indoor use 9v 105-125 lumen works very well and has minimum reflected light from walls/furniture.  It also provides the best run time, lumen ratio (when referring to Surefire 9v products).  I prefer a handheld Gladius in this application personally.  Outdoors in a rural area with vast open spaces, you can get away with the biggest and badest light your wife will let you buy.  I think a handheld M4 from Surefire would be plenty and less on the pocketbook than the M6.  An M3 with a turbo head will also help the throw on the beam over a standard M3.  If it is strictly weapons mount you are considering, a 9v Surefire with a turbo head may be a good choice.  Using the MN11 bulb you will get 225 lumen, and again, the extra throw of the turbo head for rural use.  I hope this isn't more confusing than helpful.  If you have the money, buy more than one and use them as you see fit.  Otherwise, get the best all around performer for your use.

The 9 volt surefires are just about the best package for outdoor use IMHO.  As mentioned, the Turbohead 972 will give you more throw with the same run time.  Me, I am not a big fan of the "high output" lamps as they reduce run time to the point where I worry about battery life too much.