While I'm not overly familiar with OpenStack I'd go with Ubuntu given that CentOS is being discontinued. Rocky Linux might be an option sometime in the future, which is a project started by the founder of the CentOS project.
I believe Canonical is going to be around and supporting the platform for at least the lifecycle of your system, whereas CentOS would mean that you are possibly going to be left unsupported after a certain date.
Personally, I haven't had a bad experience with either CentOS, or Ubuntu for the last 5 years, so the choice of distribution hasn't been overly significant and comes down to preference.
Cheers Dean
I've been providing software for the Navy that was intended to run on CentOS 7, which is the LTS (long-term support) version for several more years, with CentOS 8 having a lifespan ending this year. However, some companies (namely Microsoft) seem to be moving on from CentOS 7 to 8, at least with VS Code which dropped support for 7, in favor of 8, which leaves CentOS users in a horrible mess for shipping enterprise products. As a result, I've started moving to Ubuntu 20.04 LTS as the target environment, where possible. Support for Fedora 32 ended *today*. The Linux world is going through a bit of fast changes currently and I suspect the most stable place to be us Ubuntu 20.04 LTS, which will be supported into April 2030.
Hopefully Wayland will be an acceptable windowing system by 2030.