Stainless steel grill:
Are you going to make a dish below 130 degrees? If so, preferably use a stainless steel grid. The moisture that then often comes out of the product on the grill makes it stick to the grate faster, and you don't want that. If you are going to make a dish above 250 degrees such as a pizza, we also recommend using a stainless steel grate instead.
Cast iron grill:
Cast iron provides more constant heat than stainless steel and conducts better, making it ideally suited for grilling. It will get you the perfect grill marks. If the grate is properly seasoned, it will actually prevent your product from sticking.
However, cast iron does require some extra love & maintenance so you can enjoy it for longer. Thus, you need season it occasionally. You can find more information about this on the maintenance page.