"Well you’re looking through the catalogue and weighing up the best pipe and price. It isn't that simple. I only use plastic pipe when it’s out of sight i.e. under floors or built into walls/ducts, as it’s easy to thread through joists/studs etc. Stick to copper when its surface mounted and will be seen or possibly use straight lengths of plastic pipe, but clip well. Can't use plastic for the first 1m when connecting to boilers. Also remember plastic is an insulator so think about cross bonding of metal pipes if you are inserting a plastic section. PE-X pipe may be cheaper but has a mind of its own when rolling out, just ask ZEBEDEE. Polybutylene is easier to use, almost stays flat so easer to work with, reduces 1st fix installation time and is less likely to be stolen as it has no real scrap value."