Weber does make a Porcelain Coated cast iron grate for the Genesis that is sold exclusively at Home Depot. It's not heavy SS, but has lots of mass and the Porcelain coating should keep it from rusting for a long time.
I have used every kind of grill grate and I like Cast-Iron the best because of the searing ability and strength. Even though they cook the best they are not the easiest to maintain. Mine will rust if you burn the seasoning off of them, so you have to keep it out of the heavy flames. I think the Porcelain coated cast-Iron ones would be the best combo ever, but heavy SS bar is the easiest to maintain and is the next closest on a good sear. I just put the SS ones in the oven on self-clean to clean them up.