Yes Metal is usually more durable but so long as the plastic/polymer is high quality, it should be durable enough to last a long time, even with intense use. I prefer metal personally. Depends, pistol or rifle? Pistol, gas all the way. Rifle, comes down to preference. gas is cooler and easier to fix if broken but is usually quite a bit more expensive than electric. Electric is cheaper, can use highcap mags(if you don't care about realistic combat), and you don't have to constantly buy gas. Dunno, never really looked into AKs, but if you can find one made by KWA(don't think they make one but you could check), it'll be a quality piece, everything I've seen from them is.