I personally would not want plastic sights on a battle rifle. Spend the extra money for metal.
If you are going to run an optic as you main sighting system, get the Magpul MBUS.
While some others may tell you to get the Troy's or XYZ because the are "metal" and not "plastic", just how often are you going to use these sights if they are a "backup" anyway?
Also you'd be surprised to learn the MBUS held up better under abuse than the more expensive "metal" sights.
I was once asked "why" I had no problem spending $900 on an optic and then "cheaped out" and bought a MBUS rear sight.........put your money where it matters.