Question: Why are some brake caliper pistons plastic (phenolic resin) and some are steel?
Answer: The plastic material is actually phenolic resin. This high strength man-made material has several advantages over steel brake caliper pistons. The first advantage is corrosion resistance. The material will not react with water and salt and rust. But, if the brake fluid is acidic, it can damage the piston over time. The second advantage is heat resistance. The phenolic piston will not transfer as much heat to the brake fluid when compared to steel pistons.
When the engineers design the brake system they design the system with the piston material and brake pad in mind. The package of the piston, shim, backing plate and friction material are engineered together. If the original caliper piston was phenolic, the replacement caliper needs to have a phenolic caliper. If you buy a replacement and it has a steel piston instead of a phenolic piston, return it.