I work at a small resort in norway and we have 4 pvc tubes, and they cost way less. They are quite beefy. About 5cm thick. They are labled as "blems" but there is really nothing to notice. They cost 500 Nok (Norwegian Kroner) each meter. Its stupid becaus you can only get them in 6 meter. And a normal 6 meter rail would usually cost 15000 Nok.
They are more durable than metal tubes or tankers, as they don't get dents.
They slide good but i like the feel of metal. And i tend to hook a lot more on tubes.