Piston,
Piston is great in the since your Upper reciever and BCG stay clean. All of the Gas is expelled through the piston area. The bad par about that is if you dont keep theose working parts clean in the pison and lubed, they will seaze up......
Direct,
Stoner assembly since NAM. Proven out over many many manymyears. It went through minor changes over the years. The only bad thing about this is the upper and BCG get dirty and you should lube lightly after every range time or hunting. A light coat go's a long way.
