In contrast, Snappy aims for much more "Package any app for every Linux desktop, server, cloud or device" https://snapcraft.io/
I cannot find how many packages they have currently. I hope that FlatPak has more than the 11 applications listed on the website. http://flatpak.org/apps.html
FlatPak does not provide a central repository, while Snap has https://uappexplorer.com/. After install flatpak, it takes three (!) commands (on cli) to install LibreOffice. http://flatpak.org/apps.html
Snap has stuff like pulseaudio (audio daemon) or ogre (rendering engine) in the repo. That seems weird, because these are not applications. The first belongs into the base system, the second into a development environment.