For me it comes down to provenance. I need to be sure that my software is coming from a trusted source.

I originally favoured Appimages because I could get them directly from the developers. However updating Appimages and integrating them into application launchers is a manual process.

Snaps aren't perfect. They are slow to install when compared to apt-get, they consume a lot of disk space and they clutter the output of the mount command.

However I took my list of favourite Appimages and looked at which developers directly manage their own software on the Snapstore and Flathub. 

Most of the Appimages I use are also available directly from the original developers on Snapstore so I migrated to Snaps.

There are too many Flatpaks managed by random people and not enough 'Verified' Flatpaks.

I avoid Snaps that aren't managed by either Canonical or the original developers.