Flatpak is fast, lightweight actual open-source and provides security via isolation. Snap is the usual BS Canonical tries to get people to use, has a ton of bloat.
Personally I’ve had zero issues with Flatpak under Debian for desktop usage. It integrates nicely with the GNOME Software “store” and allows you to get the latest and best of everything you might need without polluting your system. Flatpak solves the usual complaints about Debian only having “old” software - allows you to run the latest and greatest while keeping a clean and rock solid Debian system underneath.