If you can afford an ACOG get one. Best thing I ever bought for my AR. I've got a TA101NSN that I paid like $600 for (just had to jump at the deal).
If I was buying exactly the one I wanted I'd have gone with one of the TA31s (my TA101NSN is lighted with tritium so someday I'll have to replace the tritium ... the TA31s use the fiber optic in addition to the tritium so even when the tritium burns out you'll have a lighted reticule, that and you still have a lighted reticule in bright sunlight).
But I'm immensely happy with my ACOG and will likely buy others when I add rifles down the road.