Sapim makes nipples in nickel plated brass and in aluminum. All nipples are forged. Arguments in favour of a brass nipple are price and rigidity.
The strength/weight ratio is the principal argument for a 7075 alloy nipple. It is superior to all other makes. If a coloured anodized alloy nipple is selected it will guard against corrosion as well as look special.
Please note, 12G nipples supplied by Sapim have a thread pitch of 50tpi