I run Debian 13 Stable with KDE Plasma, and have Flathub enabled on KDE’s Discover software store. I have a slight idea of the difference between the two: that Debian packages share system libraries and are therefore lighter in storage but do require password to access those shared libraries and could therefore become a security hazard if installing from an untrusted source, while Flatpaks have all they need and don’t require password to install, being more secure that way, but, as a consequence, consume more storage. Also noticeable is the fact that, for some programmes, the Flatpak version tends to be more recent and it therefore becomes the obvious choice when looking for the latest software.