Debian's packages are generally older and more thoroughly tested. Debian also didn't customize things as much as Ubuntu does, so the software you get is much closer to what the "upstream" projects originally released.
Debian is famous both for having extended support for all releases, and for obsessive attention to seamless upgrade-in-place support. Some people will keep a single Debian install running for a decade, upgrading as they go without ever reinstalling.
Ubuntu is fantastic if you want a user-friendly desktop system out of the box. Debian is really nice for systems that you want to run with minimal maintenance for years at a time.
One nice thing about Debian is they don't try to push snaps on you. Nowadays you can also use Flatpak get get more up to date apps on it.