I don't claim to know the answer but flatpaks have easier distribution (flathub), package management, sandboxing, can share runtimes and are also portable in the sense that they work across linux distros. AppImage is a simple and even more portable format but not much else so I guess it's superior if you only want to maximize portability.

It's depends, flatpak is a lot more portable, it bundle everything, the libraries are shared, thing that appimage can't, it automatically updates, it's more secure, and have a stronger sandbox, and it integrate better with the system