What are you looking for, and what direction do you want to go with it? Rifle, Mid, or Carbine? Varmint upper, SPR, M4gery, etc.?
I have 3 BCM uppers that fit like gloves onto the 2 RRA lowers that I own, as well as on my 2 BCM lowers. The BCMs fit too tight on my Bushmaster lower. Check out BCM, I confess that I'm a bit of fan boy, but good stuff is good stuff. Better quality and specs than RRA, but I have heard some say that the fit and finish is nicer on RRA. Maybe. Just depends on what you value more. The choices from BCM are also a bit more limited than what you can get from RRA as well, but BCM is getting better in the variety department. You can also pick up a LPK and a stock/receiver extension kit from them while you're at it.