Let's do pro's vs cons. Why do you favor one instead of the other? I'll start with... I favor Debian ISPconfig's because of their stability.
I prefer Ubuntu as it is easier to setup on most machines. It is Debian derivative so no much different from Debian itself.
I used to be Debian Developer, so Debian GNU/Linux is my favorite. Plus I trust upgrading to next release with Debian more than with Ubuntu. Ubuntu is not bad, and stability with Ubuntu is pretty good also, since I skip the not LTS-releases.
I guess both are working quite well and are very stable. but personally, I prefer Debian. Why use a derivative, when you can get the original For me, Ubuntu sometimes has the tendency to try out new technologies in the core and then abandon them later and as I run and upgrade most of my servers for many years, I prefer to have an easily upgradable and solid foundation.
