Only use copper pipe with soldered fittings if access is going to be difficult in the future. The only time I would ever consider using plastic is if there are no joints under the floor and copper could not be used.
and keep the number of joints to a minimum - soldered joints, if done properly, will be fine and will last years without leaking but it stands to reason that the fewer joints you have the fewer potential failure points you have. consider using a pipe bender where possible instead of soldered elbows etc and plan the route of your pipework to try to accommodate this.
as has already been said, stay well away from plastic and definitely stay away from pushfit for any buried/inaccessible pipework.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!