Located on the bottom of the camera, the F9 has a mechanical switch that allows you to cock the shutter without advancing the film. This lets you shoot two (or three) images on the same frame of film. For creative street photographers or lo-fi artists, this is a massive selling point that the M35 simply cannot do.
Note: Both of these cameras are manufactured under license by various companies (like Retopro) and sold under the Kodak brand. They are nearly identical in internal mechanics, but differ in build and features. In the renaissance of analog photography, two plastic fantastic cameras have dominated the entry-level market: the Kodak Ultra F9 and the Kodak M35 . At first glance, they look like siblings—both are lightweight, reusable, and designed to look like disposable cameras but last forever.