Polishlinux offers a massive comparison of the two systems – and it looks like Debian wins
See it here
(7) Ubuntu is based on Debian, which is one of the most stable and mature distributions available. Still, Ubuntu comes with fresh software and instabilities may occur. Kubuntu on the other hand is considered a bit worse in terms of stability since recent versions of KDE usually break more often that fresh GNOME.
(8) Debian is one of the oldest and most mature Linux distributions. It has a stable and large community of developers, supporters and users. If one needs a mature and well-tested system, Debian Stable (and with a little more caution, Debian Testing) is the right choice.