There are several different models from Eotech. The reason for different battery sizes is to cut down on size and weight of the unit when it's used on applications like pistols. The bigger units (longer) take AA batteries that last longer, but again, are bigger and weigh more. I wanna say the smaller unit takes N batteries. I forget. Their sights were pretty much unchanged for YEARS, then they came out with the gaurd that goes around the outside, followed by a change in the reticles available, and an option for the unit to work with night vision. That was about the same time they went to bigger / easier to get to buttons, and came out with an entirely new low profile model for shotguns.
About 4 years ago they went from being a "non-profit" government supplier (also supplied Bushnell with "Holosights") to being purchased by L3 Logistics. Don't know how much of an impact that's had on quality or anything.
I've got a unit right now that is essentially unused, immaculate. Think it's a model 522 without looking, but it takes AA's, and has the steel gaurd on the outside. No use for it, just sits on a shelf in my safe. Anybody want it for $250?