I often get questions about which RMR a buyer should add to their pistol. There is no easy answer. It depends on what the end user wants as each has advantages and liabilities. We can divide existing Trijicon RMRs into three groups.
First are the Dual Illuminated units. These are comprised of the RM03 (13 MOA Amber Dot), RM04 (7 MOA Amber Dot), RM05 (9 MOA Amber Dot), RM05G (9 MOA Green Dot), RM08 (13 MOA Amber Triangle), and RM08G (13 MOA Green Triangle). They are all within a few dollars of each other, but the Dual Illuminated are the lowest cost RMRs. They can be identified by the grey plastic shroud that wraps around what some call the "cat ears" on the RMR.
The advantage of these is that they will never need a battery since they run on fiber optics that gather existing light and fuse it onto the dot or triangle. In reduced light where the fiber optic has collected nothing, an internal tritium element powers the dot. For some of our customers that tend to travel in the Third World where batteries are simply not available, we suggest this option. The fiber optic is a passive unit that will last forever, and the Tritium should last a decade or so.
This is not without disadvantages however. We have found that if shooting from a darkened environment into a lit environments (say a dark room into a lit hallway...or from a darkened house out into the sunlit exterior), there is an effect called washout. What happens is that there is not sufficient light to activate the fiber optic, and it is not sufficiently dark to activate the tritium. The dot does not disappear, but it is quite faint.
As well, when operating with a weapon mounted light, or any of the flashlight shooting systems, as soon as the flashlight is turned on, the shooter will experience that washout effect. This is of course immediately nullified by the shooter transitioning to the iron sights seen in coincidence with the dot, but the effect can be a liability under the mentioned conditions.
Second are the two LED Constant On models, the RM01 and RM02, in 3.25 MOA, and 6.5 MOA respectively. They are identified by no grey plastic shroud and instead slick black sides. These are constant on all the time and have an internal brightness monitor that increases or decreases brightness according to ambient conditions. This is a great feature and most will find this a "best buy" unit in either MOA. The batteries will last a good long time, but we recommend they be changed once a year. Pick a date and change the batteries.
One thing that I have experienced with the Constant On LED Models is that in the mile high AZ wilderness, with the ultra bright sunlight, oftentimes the RM01/RM02 is not as bright as I'd like it. But that is the only drawback, if it can be called that.
And that brings us to the Adjustable Models - RM06, RM07, and RM09 (3.25 MOA, 6.5 MOA, and 1 MOA respectively).
These are like the Constant On LED models and share all their characteristics with the addition of an On-Off Switch that resides on the sides of the RMR. This allows me to increase the brightness or to decrease it. I will refer you to the manual for details, but suffice to say that I can increase the brightness to a high level so it is exceedingly visible even at high noon on a cloudless day at 7,000 feet. Yet, I can also make the dot nearly invisible unless I am using my PVS-14 night vision unit. Similarly, I can simply shut the unit off for travel, or for iron sight training should I wish to do so. These are the most costly of the RMR family.
That is it in a nutshell. Choose the one that works best in your situation.