[Drakker]

Simple answer, plastic pistols are lighter and require less force to cycle, they are more durable and reliable, although a good brand helps (hint: Tokyo Marui).

Metal pistols are heavier and take a lot of force to cycle. This puts a lot more stress on the parts. Also, almost all manufacturers use low quality metals, which will break, deform and wear out. They kick harder but are not as durable. High quality metal upgrade parts exist, but most of them are made to Tokyo Marui spec to replace the plastic parts and might not fit all that well on clones.