Answer: -How cold does it get where you live? If you put plumbing in the attic it will be exposed to freezing, similar to plumbing in a crawlspace (only worse). -Copper is old, pex is new -- relatively at least. Pex may perform as hoped or not. Time will tell. I would go with copper, around the pipe in the cold attic I would place an air plenum that circulates air from the house around the pipe. Insulate this air plenum or duct from the attic cold. If you use pex it will freeze nearly as quickly as copper. But THEY say it will survive? But YOU may get thirsty before spring. I would put an insulated house air communicating chamber around the pex also. Maybe the pex manufacturers will add some carbon to the plastic -- so we can put a terminal connection at each end of a cold run -- a thermostat would program resistive heat energy electrically to the pipe.