Roof construction - Which type of vapor barrier?

Even gypsum plasterboards still need to be worked on. Not as extensively, but still. And a foil is relatively prone to errors. OSB must then be glued accordingly in tongue and groove and the joint to the wall must be provided with permanently elastic strips. Both have advantages and disadvantages here. I would choose OSB.

What else do you want to work on the surface of drywall, aside from priming? I myself have a kitchen island clad with OSB and can tell you that quite a bit of work has to be invested before it can be wallpapered with painter's fleece.

A foil is significantly less prone to errors in execution. If there is a hole in it, you can see it. The tightness of the tongue-and-groove connection seems secure at first glance, but on closer inspection, it is not, because the tongue and groove are not always worked equally well or get damaged during transport. Then you come to the conclusion that a leak in the connection can only be detected during the BD test or similar. Furthermore, the wall connections with so-called "permanently elastic joints" are certainly more prone to errors than a plastered foil and above all less durable.

Spackling and sanding are also part of drywall panels. OSB naturally requires more attention. If the permanently elastic joints should be problematic, that is also the case with windows. If you use the swelling tape installed there, it certainly works as well. And one probably also pays attention to the flawless grooves and tongues. No one installs defective panels. However, your arguments certainly have merit. Nevertheless, I would recommend OSB. In terms of mass, it absorbs sound considerably in my experience and provides good heat protection. By the way, I myself covered my attic with OSB instead of foil and can speak from experience in this regard.

Oh, apart from that... It is being carried out by a company anyway. So the additional effort doesn't matter as long as it doesn't significantly affect the price.

A vapor barrier is intended to prevent moisture from the living space from entering the insulation so that it cannot mold.

Do OSB boards replace a vapor barrier? I have never heard that and can hardly imagine that OSB boards are vapor-tight themselves, but I am happy to be proven wrong.

There is still a difference between a vapor retarder and a vapor barrier.
Additionally, there is summer (warm outside = insulation warm) and winter (cold outside = insulation cold).

If vapor from inside (rather warm) reaches the insulation (when warm), not much happens; the vapor passes through the insulation to the outside and the insulation dries out.

If vapor from inside (rather warm) reaches the insulation (when cold), the vapor condenses and the insulation becomes damp/wet and cannot dry out afterward because of the cold. Mold can form.

The "climate membrane" seems to be the optimal hero here. When it is warm outside, it is "automatically" vapor-open (more like a vapor retarder). When it is cold outside, the membrane acts as a vapor barrier. In total, this means (at least theoretically) a lower risk of mold. As far as I know, a climate membrane costs less than one euro extra per m². Installation involves the same effort.

We have Intello Plus in our range. When I search online for climate membranes, I find many offers from Isover. I cannot evaluate either. Building material dealers can provide (dependent) advice here.

Edit:
Vapor protection is only necessary where vapor is generated (people) and the insulation must be protected.
OSB boards as flooring in the attic space have little impact here because they are installed above the insulation.