Reliability. Ammo availability. Magazine capacity. Parts availability. I know all 3 are powerful. But reliability to feed and function is high on my priority list, so is the availability issues not necessarily today, but 5 or 10 years from now. Tell me what you can please. Because I'm not experienced with any of these I would probably opt to buy a complete factory made upper, and not try to assemble and test myself. Unless I'm convinced it's pretty straightforward to assemble one.