There's no reason to use Ubuntu for this scenario. Debian will be cleaner, simpler and easier to support long term. Ubuntu complicates things and there are no benefits to compensate that for OP's use case.

Debian is (rightfully) known for being lightweight and very stable. Particularly with older hardware, while still being quite compatible with newer hardware. Their long-running release cycles tend to not break whenever updates do roll out. Ubuntu is Debian based as well, its focus however is on user friendliness and usability, especially on the GUI front. Ubuntu server is perfectly fine, but it’s heavy handed compared to a minimal Debian installation with just a handful of packages selected purposefully by the user for the task it is intended for. There have also been more vocal complaints about whatever Canonical is trying to do with snaps/snap store.

Most beginners with Linux I would more encourage to try Debian for its stability and speed because it’s a great platform to learn Linux on as well as experiment with whatever goal they have by way of packages and projects available all over the open source side of the web.

Honestly, I would say because you just have less struggle. I had just a lot more problems when I was using Ubuntu instead of Debian. But I thinks it's mostly personal preference

I agree w what others have said. Just some personal background: started with Ubuntu (well technically I started with Suse, then then Ubuntu). But I found on the server, Ubuntu required a lot of manual upkeep. Debian was just always worked, never had to think twice about it.