I installed both and can't decide what to focus on. Which one to make my primary OS.

I work with redhat daily and it's nice to have a redhat-compatible OS on my personal machine.

OTOH, Ubuntu seems to be the most popular Linux ever with the largest market share and I am trying to decide if I should include that in my boot options.

The biggest difference that I identified is the much different way patch (update) management occurs. yum, rpm in redhat types and apt get in Debian-derivates.
Are there any other key differences I need to be aware of?

I choose my distro(s) by the package manager, almost every distro that I have used since my time as a beginner trying out, (distro hopping), various distros has used apt, it is what I'm used to, I suggest you find your package manager of choice too.

Between the two choices you posed, I'd go with Debian. Joking aside, for typical home user purposes, though, Ubuntu may be a friendlier home-user distro in terms of the available applications in its repos.

But it appears to me, based on what I quoted above, that you have a good reason to use Oracle, or perhaps ALMA Linux, which a CentOS fork in response to RHEL's changing CentOS to sort of RHEL testing. You'd be learning stuff at home that you can use at work, and vice versa.

Just a few thoughts.

I ended up doing both. An install on a SSD of Oracle 9.1 and a liveboot of Ubuntu's latest-greatest.

Oracle Linux is mainly a server operating system. It's a tiny percentage of Ubuntu's user base. I was aware of that. In fact OL8 did not even have wi-fi support. I complained and they rolled it in in the OL v9 release. So I am happy. It runs well on my laptop.