Question Author
Thank you very much The Builder, that's very helpful.
There is another reason I had for using a wiper over a tube, but I wanted to see what other people thought. If the tube becomes dirty inside, it can become more or less impossible to clean. Our shower screen was fitted with a tube seal, so when it became dirty I had to buy a new one, whereas if it had been a wiper seal, I would have been able to clean it. That's why I was wondering whether a wiper seal would be less effective than a tube seal. As Homebase only had the tube type, that's what I replaced it with, but if it needs replacing again I'll look elsewhere for a wiper one. As Homebase only had the tube type, I thought that they must be more popular, for some reason.
Actually, when I was replacing it, on close inspection I realised that the top of the bath and the wall to which the shower screen is attached are not quite at 90 degrees. This means that the tube seal is more squashed at the end furthest from the wall. So the seal is effective, but moving the shower screen requires a little effort. That's when I started to wonder if a wiper seal would be more tolerant, so I was thinking along the same lines as The Builder.
The shower screen wasn't fitted by me, by the way. I know if you were fitting a shower screen in the first place you would be able to drill the holes in the metal U-tube to compensate for a not quite vertical wall (up to a point). But I don't know if there's any way to fix this now, without getting another metal U-tube.