I've skied for a long time and I can't say I recall seeing too many metal signs on ski slopes. I see a lot of wood signs and a lot of coroplast signs. A LOT of coroplast signs.
Remember, if someone can hit it, they WILL hit it. It's be a bad thing to have someone rip open their head or face on a metal sign. I know it's not "sharp", but if you crash into the edge face first, it's going to be ugly.