Since I've given up on using Linux on the desktop, choosing Debian is pretty easy for me. Ubuntu Server doesn't provide me with any features that Debian stable does not, and I quite like the extremely conservative nature of Debian for my simple uses.
On the desktop, I would still prefer Ubuntu (or Xubuntu/Kubuntu/...), mostly just for being so extremely mainstream. If you need something stupid to work, be it binary blob drivers for graphics cards or experimental research projects, chances are someone decided to either package it directly for Ubuntu or they made a how-to specifically for Ubuntu. Distributions like Arch and Slackware provide this strangely satisfying experience of setting up your system exactly how you want it, but continually tweaking the system is a very addictive time sink.