Check out M4Carbine.net for everything you ever wanted to know about ARs. The folks on that sight are mostly military and LE people that have a great deal of field experience using and fixing the rifle. They favor Aimpoint T1 or H1 for a red dot (four year battery life and mil-spec quality, minimal failure rate). Trijicon TR 24 for glass 1-4 power or if you are rich one of the Schmidt & bender short dots. Stick with American Defense or Larue Tactical mounts for either. Troy back up sights are pretty much bombproof.

I had an Aimpoint M2 sight on my department issue M4 for nine years of fairly heavy use in Alaskan weather and loved it. It took a beating and kept on glowing. The long battery life is great and the newer models of Aimpoint have new technology dots that run many thousands of hours before battery replacement. I currently use Aimpoint T1s on my social carbines.

Check out G&R Tactical for combo mount/sight package prices. Brownells is a good source too.