This is just me, and my way of thinking, but, I would do this: Of all the AR-15 style firearms now being offered, I would get one that you could use any of the aftermarket AR-15 part in it.
Some of the AR's now being manufactured by large gun companies, are beginning to use proprietary parts and I feel that that limits their usability. For me anyway, it sure seems like a very limiting factor.
I just bought a LWRC AR-15 carbine and even though it was expensive, I like it immensely.
Bill