I have several AR's, as well as a number of AK's and SKS's,so I'm familiar with both calibers. What's the intended purpose? Both are a solid self defense round and will do the job. If you plan on hunting deer with it go x39. Yote and smaller..I'd go .223. If it's going to be more of a gravel pit gun to plink with and you do not reload, I'd go x39 just because of the cost of ammo. A case (1K rounds) of .223 reloads goes for 380 up here at the gun show. You can still get Wolf or Golden Tiger cases of x39 for about $225.