That's a great question, and one I get a lot from folks looking to buy a new outdoor bin. Having used both types over the years, I can share a pretty straightforward comparison based on durability.
Metal trash cans, typically made from galvanized steel or aluminum, are the heavy-duty champions. Their biggest strength is structural integrity. They can handle bumps, won't crack in freezing temps, and are much more resistant to animals like raccoons or stray dogs trying to chew their way in. A good powder-coated or galvanized metal can will stand up to sun exposure for years without fading or becoming brittle. The main durability concern with metal is rust. If the protective coating gets scratched or chips away over time, moisture can get in and cause corrosion, especially around the bottom. But with decent care, a quality metal can can easily last 10-15 years or more.
Plastic bins, usually made from polyethylene, are the lightweight contenders. Their durability advantage is in their immunity to rust and corrosion. They're also great at resisting dents. However, plastic has its weaknesses. Prolonged exposure to intense UV rays from the sun can make the material brittle over time, leading to cracks, especially in colder weather when the plastic contracts. They can also become warped or misshapen if exposed to high heat (like next to a grill). While sturdy, the walls are more susceptible to punctures or being chewed through by persistent animals. A well-made, thick plastic bin can last a solid 5-10 years, but it often shows its age through fading and brittleness sooner than metal.
So, for pure, long-term durability against the elements and physical punishment, metal generally wins. It's the "buy it for life" option if you get a good one. Plastic offers very good durability for the price, with excellent rust resistance, but its lifespan is more limited by sun exposure. Your choice really depends on your climate, budget, and how much wear and tear you expect.