FWIW I've talked drunkenly about and weighted the two options many times.
Metal or plastic pulley to me doesn't matter IMO. It's all about the bearing inside. If that bearing starts to go, the pulley has an issue and you have an issue.
Plastic pulley if the bearing locks up will stop and the belt will melt it fast. Likely resulting in you needing a belt and pulley.
Metal pulley if the bearing locks up will stop and you will smoke the belt pretty fast. Resulting in you needing a belt and pulley.
Moral of my rambling is.. which is actually better or worse? Notice the squeak and replace the pulley before you kill a belt.
