Choose Debian. Real secret reason? Debian means you have to upgrade to new versions less. Ubuntu LTS lasts as long as Debian (5 years) but they crank out a new LTS every 2 years. If you have 3rd party sources, I’ve run into they only support the two latest LTSes - well you have one year left on your 5 year support… but do you really? And now you can put on your sad face because you get to do two LTS upgrades. All while Debian was over there on one version and everyone will target that one version and its previous version no problem.

Debian just makes better sense for a server.

(And Ubuntu is stupid with its “ooh extra patches just register your machine with our central authority” ok bud my Debian machine just says I have mail for some reason every time I boot it up, it doesn’t make it weird and gimmicky and based on a “free” deal that could change whenever they want to jerk you around)