re: AR advice

Use the search. There were about 3 AR15 threads in the past week.

All of them will run just fine. It's really about what level of accuracy you want, and how much you want to accessorize. I have a DPMS 20" upper on a RRA lower and it shoots amazing groups. But then again, it's heavy as sin and a 16" would be better for moving around.

Decide what purpose you want it to fill, and then you'll have a better idea.