Plastic is cheap and lightweight, but in my experience, plastic pedals die an early death. There are "nicer" plastic pedals out there, and i don't have a lot of experience with those, but very cheap plastic pedals aren't even worth mounting on a bike, even if you got'm for free. Super common bike shop scenario is where a rider (usually a high school kid) comes in holding what remains of the platform, and tells us he rode to the shop on the spindle after the thing broke. It can happen with steel/mag/aluminum pedals too, but the metal pedals last a lot longer.