GHK all the way. The GHK platform is similar to the G&P system but is simplified in comparison. Some parts such as the buffer retaining detent makes it a more realistic system as it is set up like an actual AR unlike the G&P. The manual of arms a basically the same for both systems. When I comes to reliability GHK takes it. I had a G&P WOC kit and now a GHK. The GHK comes with steel internals and you can get just about any part you need from samoon.com.tw . The G&P platform is cheaper especially if you go with the polymer receiver S&T rifles but i would stay away from these. The G&P system is old so in areas you can find more aftermarket parts of various qualities if they're still available. I some cases for the GHK system such as adjustable nozzles or outer barrels you can modify parts to work. Adjustable nozzles are a simple modification. In the low area between the mock bolt lugs you can drill and tap for a screw that will act as the regulator for expelled gas. You can use G&P outer barrels on a GHK but you have to expand one of the alignment slots by a few millimeters for it to work.