Spring airsoft rifles are the most reliable, but the drawback is that hey need to be cocked before each shot. This makes them a good choice for airsoft sniper rifles, but not usually as assualt rifles. Usually spring airsoft rifles are used by beginners. You will not find many spring powered rifles used among experienced airsoft players, unless they play the role of a sniper.
Spring airsoft rifles are maunually cocked before each shot, which means there is no other power source required. As long as you have ammo you stay in the action. No chance of batteries running out or running out of gas.