Which BBQ grill is better: cast iron or steel
Assuming you are talking about different types of grill grates:

Choosing Grill Grates

In general, cast-iron gives you better heat retention (flip-side: slower preheating) and better non-stick behavior if well seasoned. However maintenance with cast-iron can be a bit of a hassle. You have to develop and maintain seasoning and also take precautions against rust.

Stainless steel does not rust and is easier to maintain, however it is less non-stick and won't give you the same grill marks that you get with cast-iron.

Ceramic coated cast-iron is supposed to be a good compromise, however, I really hated the ones I had as they were very hard to clean and the coating eventually cracked, rusted AND chipped off.

tl;dr - If you are willing to put in the maintenance, cast-iron is generally the superior choice. If you aren't, go for high quality stainless steel grates (like these) -- you won't be missing all that much.