Zip is flexible, button is rigid
Skinny jeans always use a zip fly. On 100% cotton jeans, depending on the style, the fly can be either a zipper or a button fly. A button fly used to be thought of mainly as a men’s-jeans thing, but these days it’s used even on women’s jeans — and honestly, the men’s/women’s divide in fashion is closing up more and more anyway.
With a button fly, the front rise comes out to the same measurement regardless of size. With a zipper, we have the option of using a shorter zip on smaller sizes, but as long as there’s enough room in the front rise, there isn’t much difference between the two. It used to be that the zipper-versus-button choice on the fly varied from country to country, but that’s no longer really a thing.
Bottom line: if you’re undecided, a zipper is always the safer. And it’s not something that affects sales one way or the other.