I carried an Aimpoint on a M4 while in Iraq. Good sturdy unit, easy to sight in and use. I have an EOTech 511 on my personal M4 (Bushmaster). I prefer it over the Aimpoint as I like the sighting reticle. To me it's a matter of personal preference; both are high quality items.
P.S.-The difference in the EOTech 511 and 512 is the batteries. The 511 uses N cell, the 512 uses AA.