That's an excellent and very practical question for anyone managing public spaces. Having dealt with this exact issue for municipal projects, I can tell you there's no one-size-fits-all "best" material. The winner depends on your specific priorities: extreme durability or lower lifetime cost.
Let's break it down. Heavy-Duty Plastic (like HDPE) is the modern workhorse. Its biggest advantage is incredible resilience against the elements. It won't rust, chip, or corrode from rain, salt (for coastal cities), or harsh cleaning chemicals. The material is also lighter, making installation and moving much easier. Modern plastics are UV-stabilized to resist fading and are often made from recycled content. The main drawback? It can be vulnerable to extreme heat (warping) and determined vandalism involving fire.
Metal (typically powder-coated steel or aluminum) offers a classic, sturdy feel and superior vandal resistance. A well-made metal can feel more "permanent" and can handle rough treatment and very hot items. However, it is prone to rust if the powder coating is scratched or chipped, requiring maintenance. It's heavier, more expensive upfront, and can get very hot or cold to the touch.
So, which holds up best? For most general urban settings—parks, sidewalks, downtown areas—high-quality heavy plastic often provides the best balance of durability and low maintenance. It handles daily weather and use superbly without rust worries. Choose metal if your primary concerns are extreme vandalism, very high-traffic areas where a "heftier" feel is desired, or for a specific aesthetic. Always look for commercial-grade models from either material, as residential units will fail quickly in public settings.
My final tip: Don't just consider the bin. Look at the hardware (lids, hinges, bolts). On metal cans, these are often the first points of failure due to corrosion. On plastic cans, ensure they use thick, reinforced areas at stress points.