Hi I'm interested in learning more and more about each of these firearms. I'm currently stuck between a s&w mp15 and a glock 17. It will be used mainly for the range but also for HD whenever needed. The only upgrade I am interested in is a trijigon night sight for the glock. As for the ar15 I plan to do all of the necessary upgrades (optics, sling, rail system). I'm confused on how customizing works on the AR15, are there any websites or books that go over this topic?