I don't think Debian needs to choose. Debian will keep using deb/dpkg as flatpak and snap are not intended to replace a distro's main packaging. They are supplements so users can get newer packages directly from upstream developers. Note that Debian 9 already supports both flatpak [1] and snap [2]. Because of the way snaps/flatpaks are isolated from the rest of the system, they can coexist without issues.

I thought the creators of snaps/flatpaks would be upstream developers, so they themselves will vote on the package management system by using either. There is no duplication of effort here.

Myself I prefer AppImage [3], because of its simplicity and the fact that they are already supported on all Linux distributions, without installing any explicit support.

[1]: https://packages.debian.org/stretch/flatpak

[2]: https://packages.debian.org/stretch/snapd

[3]: http://appimage.org