Snap is terrible in every sense, so you should never use it in my opinion.

As for Flatpak, I'd say use it if you need a more up-to-date version of a piece of desktop software than is in your distro's repository or desktop software that isn't in your distro's repository at all.

For example, I use the Firefox flatpak on Fedora to have the most up-to-date version (98.0.2) since the current version of Firefox on Fedora (98.0.0) was giving me some issues like crashing when downloading something and choppy gifs.

I also use it for some proprietary software like Spotify and a game called Vintage Story. Adjusting their sandbox permissions with Flatseal is useful in this case.