Stainless steel and cast iron both have different properties, which make them useful for specific functions. Although stainless steel is more expensive than cast iron, it has poorer functionality when used as a grilling grate. This is due to the differences in how stainless steel and cast iron conduct heat. Although stainless steel cookware is lighter, and has better protection against corrosion, cast iron can be treated to have the same characteristics.
Iron and Steel
Both cast iron and steel have carbon atoms diffused into the metal. This happens during the iron smelting process, when carbon is absorbed in the furnace. Cast iron is cheaper to manufacture, because the molten iron is poured from the furnace directly into a casting mold. Steel must be re-smelted and refined to remove excess carbon ("steel" can only have a carbon content up to two percent.) Stainless steel is an alloy of steel with other metals, typically chromium and nickel.
Iron and steel have a crystalline structure of iron atoms interspersed with carbon. This gives the metal its strength, as the carbon inhibits molecular slippage when the iron is subjected to stresses. The higher carbon content in cast iron makes it heavier and harder. However, this also makes cast iron more brittle. Stainless steel is lighter because it has a lower carbon content. Chromium molecules in it form a protective oxide layer on the steel's surface, which protects the iron from rusting.
Higher amounts of carbon in cast iron allow it to retain heat for a longer period than stainless steel. This allows heat to diffuse through the metal evenly, which means the cooking surface conducts heat more efficiently. Stainless steel does not conduct heat as efficiently because it is lighter. But it has an advantage over cast iron because it isn't as brittle. It also is better protected against corrosion by weathering.
Cast iron cookware can be seasoned so surfaces are non-stick and are protected against rust. Cast iron pots are rubbed with fat or oil, and then heated in an oven for two hours. The fats form a protective layer on the surface. Cast iron grates are typically used for grilling, and the fats and high heat will naturally season the iron. Stainless steel cannot be seasoned because the chromium alloy in the metal prevents oils or fats from adhering to it and forming a "seasoning" layer. However, foods cooked on preheated stainless steel cookware will not stick.
Cost and Lifespan
Cast iron cookware costs less than stainless steel pots and grills, because they easier and cheaper to manufacture. They are also more cumbersome because of their heavier weight. Stainless steel is not as heavy as an equivalent cast iron implement because it is an alloy of lighter metals. Stainless steel will last longer than cast iron because it is not as brittle and rustproof. They are also easier to clean, as scouring will not expose underlying metal to corrosion. Cast iron on the other hand cannot be scoured or cleaned with detergents as this will destroy the seasoning layer.