I'd say the ease with which Debian can be kept up-to-date as a rolling distribution (just follow testing or unstable (which isn't all that unstable, breakage is rare, for that you want experimental)) versus the convoluted process of moving from one Ubuntu release to the next. Where Debian can be kept in line with apt update; apt dist-upgrade (or full-upgrade which does the same) Ubuntu generally necessitates a re-install between releases.