KJW is indeed good brand.Kjw are good pistols. Greengas is better than co2 for pistols imho
But green gas/co2 question could be answered only when you take the conditions when using it. If you live in hot climate gg is best option. If you plan to use the pistol in temperatures below 12 C degrees co2 is better cause gg will not shoot at all.
Personnaly I dont have experiences with full auto but my friends always tells me not to buy pistol with such option. They often struggled with faster worned out parts.