Plastic (resin) sheds win on assembly speed and zero maintenance: no rust, no repainting, and panels that snap together in an afternoon. They handle rain and UV well, and most come with built-in floors.
Metal sheds win on price per square foot and raw strength against impacts. They're a great choice for larger footprints on a budget, but plan for anchoring, and check that yours has a rust-resistant coating.
Our advice: if you want the fastest setup and a clean look, go resin. If you need maximum space for minimum money, go metal — and add a floor kit.