You typically have a choice of five different materials in the construction of a grill's grates.
Lower end grates are made of chrome to keep the price low. They are harder to clean and prone to rust.
Porcelain-coated steel grates do not rust easily and clean up is a breeze.
Cast iron grates can heat well and evenly, but require high maintenance to avoid rusting. If you use and store your grill outside, this is a challenge.
Porcelain-coated cast iron has the cooking benefits of cast iron grates without the cleaning and rust issues. Note that you must be careful not to scratch the porcelain off by using a brass bristle brush in cleaning.
Stainless steel grates are going to last a long time, although the trade-off is in cooking quality, as they do not hold the heat or sear as well as the cast iron models. Slow cooking meats like poultry or roasts do well on stainless steel, unlike steak or fish.
Note that the grates should be wide and close together for better searing. Thin, round grate rods don't cook as well, and food has a tendency to drop through the spaces between to the bottom of the grill.