If you talking about the infrastructure cost, then yeah it flatpak would be 'better'. since all your users would use the same runtime and you only have to ship the binary.

But this only makes sense for the first time the user downloads the application, because it turns out appimage can do delta updates just like flatpak lol

In practice not even the first point is true btw, a lot of flatpaks often ship bundled in dependencies (due to having compat issues with the flaptak runtime), so the donwload size of the .flatpak alone is similar or bigger than the one of the appimage.