When I first started using my Weber Genesis 310 (2008) I called Weber about the best brush to use on the porcelain enabled cast iron grates. The customer service person said use brass as stainless steel can damage the grates. I've used brass ever since. I've been using Libman brushes for years now. The link below on the Weber site mentions uses stainless steel brushes. Just curious what you've found regarding brushes?
A little care and maintenance will protect your cast iron grates for years to come!