I know the M21 has it's share of proponents, but the first thing I wondered about when I looked into it is what happens when the tritium fades? I have had to replace enough tritium sights to know that it doesn't take that long. I contacted someone (I can't say with 100% certainty, but I think it was someone with Mako) a couple years ago with that same question, hoping to hear that It was a simple little user replaceable tritium ampule insert or module. However, the response I got was that you would have to send the unit in as it essentially requires replacing all of the internals, and the cost, as you would expect with having to replace most of the functional components, was just short of a new unit.
Just like the arguments between the proponents of of those two battery powered sights, though, the Mepro will have it's share of users who feel like that configuration suits them better. None of the solutions are perfect, and arguing between these 3 is the equivalent of a debate between 9mm, 40, and 45.
I just thought I would offer up this technical tidbit of information, for you to make a better informed decision about which system best fits your own situation and desires. For some people, that convenience and reliability of not having to replace a battery for as long as the tritium lasts is worth the cost. Others will prefer the cheaper option of feeding batteries to an EOTech or AimPoint.