Having used both recently, I found when some vendors target just one distro to package for, they target Ubuntu because it's more popular, not Debian.

I thought "No problem, the .deb format works on both!". But in practice the Ubuntu and Debian release cycles are not aligned and so their dependency trees are not always aligned, so some vendors .deb package for Ubuntu may not work on Debian.

You can choose Debian and hope you don't run into this, or you can choose Ubuntu and benefit from current and future support from a more popular desktop distribution.