Hi, I personally would use papier mache which would give me a nice light mask which is also sturdy enough to look like metal when painted. A resin cast mask would be sturdy but heavier and a latex mask would be lighter but not sturdy.
If you do use papier mache you can either mould it directly onto a cast of your head, or a standard blank shop bought mask; or you can make the mask out of clay/plasticine, then cover it with plaster of paris and then use this 'negative mould' to put your papier mache in.
The first option is quick and easy, but may mean that the mask isn't as smooth as you want. The second option allows you to be happy with your creation first, and means that the finished mask will be pretty close to the one you moulded in clay.
Good Luck!