I prefer the cas iron with really heavy grates. You get a surface that not only get's hot, but it REALLY holds the heat. It gives good grill marks and helps get food cooking faster. Of course the problem is it needs to be taken care of. This means keeping it clean and making sure that it is well oiled over the entire surface. Grilling grease and fats will help protct it but you don't grill on the entire grate so you need to take them out frequently and make sure there is no rust and that they are well oiled. The cast iron will last for decades if you take care of it. But if you are not willing to do all the work go for a high quality porcelain coated cast iron grate. You get the heat of cast iron in a rust resistant surface. When you buy your cooking grate though, get a good grill cleaning brush that won't chip the porcelain.
Hope that helps! Enjoy!
Trish