Given the real effective range of 5.56, coupled with the typical 200 or less yard ranges to train at and the shear odds against you having to make longer than a 400yd shot in NE due to trees, I'd say the lower power ranges are the way to go. If you're really concerned about engaging targets beyond 400 yards, odds are you already own a larger caliber deer rifle with a higher power optic sighted at a longer distance.
I have one of the Sightmark's on my AR I bought at Xmas and it's held up well, about 500rds and no drift. 1/4 MOA external adjustments which is nice. Only drawback is no illuminated reticle, but for $125ish it's more than worth what you pay. If money wasn't an issue I'd go Leupold, but I'd like to build my wife a rifle before I start spending that kind of money on a scope.