Actually if the joint is made properly on a compression fitting you don't need either. A smear of hawkwhite or bosswhite (water hawk or bossgreen for potable water) round the olive helps. Wrapping PTFE round the thread does nothing for the seal but does prevent that annoying screech of metal on metal - particularly on 22mm+ comp fittings.
Its the olive that does the work so if you want to use PTFE to help make the joint a wrap round the olive is a good plan. But not too much otherwise you won't get a seal at all.
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!