There are metal corner beads and plastic corner beads(such as PVC corner bead).The advantage of metal is that it is extremely sturdy and durable, and it will last for the lifetime of the wall. The disadvantage is that metal can rust, with the rust eventually bleeding through the paint and making a rather unsightly mess. Plastic is not prone to rusting, and while it may not be as durable as traditional metal, it is easier to work with.