Different people have different results. It really is a case of suck it and see. Grinding disks can be picked up from between 80p and £1.20, so it's not a life changing decision between stone and metal cutting, buy one of each. Peoples grinders all grind at differnet speeds, people hold the rim at different angles, they move the grinders at different speeds down the rim, they go over bits of the rim twice, etc etc. You just have to try it on your rim, and see what you find best.