I went with the Aimpoint M4 from www.tatedefense.com
(best price I found). 8 year battery life and very fast pick-up. 2 moa is good to 300 yards for me. I think it is the most strongest built red dot on the market.
Plus it sits very low on the Sig 556 if you do not use the AR spacer that comes with it. It also has a great mount that is basically a built-in torque wrench. Just turn the knob till it clicks twice and it is locked tight- easy to remove too.
Great bright optics with killflash included with the one I bought.