It depends on your likes and desires. I would stay away from a LL because I see them as just a slight upgrade from the string trick, but that is me. Next would be the RDIAS. Some like them better than RR guns because you can put one in any lower you want and they would most likely survive a KB. With the variety of lowers available for different magazines that is a very real benifit if you are interested in that. Even if you are not interested in that option you may want to go that route just in case you get the itch later on. The down side to a RDIAS is that sometimes you have to tweek them to work in a given lower / upper combo, but that is a minor inconvenience. I bought a RR over a DIAS for one reason. I bought a Shrike upper before I bought an M16. The open bolt option will only work with a RR. I may beet my lower to heck if I use it, but then I own an Oly conversion, and I'll just send it back to have it rebuilt if I damage it. IHTH. Happy shopping.