Also take into considoration your budget when it comes picking the two. If you feel that you shouldn't be paying a whole whack for magazines, buy plastic, they do the job, and they feed well depending if you get the better brand (I've had best luck with STAR magazines.) And should you loose them or break them, buy more. I got a box of 10 plastic AK mags for only 60 bucks, which is a great deal for me. They do their purpose, all have been tested and feed well, but chances are that they will break, the plastic isn't perfect.
Metal, you get durable, nearly flawless magazines, weighted and sturdy. Chances are if you keep care of them they will do you good in the long run. But they end up becoming a huge investment and will be a pain should you loose one.
So, it comes to choice, would you think its worth it?