Ultimately, the preference is up to the individual photographer. And the speed of the lens is only one consideration.

The prime lens, whether faster or not, is generally a simpler design, which generally results in better performance with regard to sharpness, lack of aberrations, etc. It doesn't have to make the many compromises a zoom lens does in order to be "good enough" at both ends of the zoom range and everywhere in between, and there are usually fewer elements and other moving parts to reduce transmittance and introduce other errors. However, it does mean that you might have to "zoom with your feet" to get the framing you want.

For landscape photography, even on an overcast day, there is generally more than enough light available, except early morning or late evening, so the speed of the lens is not as critical as for indoor photography or other low light situations.

The zoom lens is for many more convenient because you can get a variety of framings of your scene without having to move much, but the additional design decisions mentioned above can result in more image issues to deal with. That said, though, lens design these days has made those differences generally far less noticeable than they used to be, so the convenience of not having to move around so much is often considered a benefit.