IMHO copper rules in most situations (unless you have low pH or aggressive water) ! Over time, the plastic can sometimes *sag* and possibly get brittle. If it needs to be repaired or altered in any way, the pipe will have to be glued and you will have no water 'til it dries. Copper-you can solder, turn it on, test it and know that all is well. Any plastic to metal connection is weak, such as where the HWT connection is made. Mice and rats love many plastics. They chew on it to keep their teeth from growing through their lower jaw.