For material that thin I'm not sure you are going to notice much difference. I'm not sure what size mills you are going to use but the prices of solid carbide mills in the smaller diameters is really cheap, like a 1/4" solid carbide for less than $8 at MSC. I think that is what I would opt for. I'm wondering though, how are you going to hold the gasket wihile cutting it? If you have trouble with the mill trying to lift the part, you might want to try a left-hand cutter, if that's an option.