I would go with copper over galvanized outdoors. You can certainly bury copper without too many worries; in fact copper is used for main water supply lines to many homes. As far as PVC cracking over the winter, it certainly becomes brittle when cold. So, if it's full of water and freezes, or if it's hit when cold, it may crack, but it will hold up well otherwise, especially if buried. A last PVC concern though, if you have any ground movement (frost heaves or settlement), that may cause trouble for buried PVC.