They are different linux distros:
Arch is the spear head. Bleeding edge rolling release: You get all updates as fast as they are coded. Because of that, your system tends to break more often as mistakes happen.
Fedora tends to get updates relatively quickly. If you are using hardware that needs new drivers, this is a good option while being more stable than arch linux.
Debian tends to be very stable, as all updates (besides critical security updates) are battle prooven. On the other hand, you have to wait a long time for drivers or feature updates. Literally years in some cases.
Disclaimer: May contain opinion!
Ubuntu is based on debian. It is more loaded with “stuff” that many ppl find good as they do not have to configure or install much to get started. Other ppl find it to be bloated/configured the wrong way.
IMO debian is better in an appVM qube setting than ubuntu. In a standalone this boils down to: Personal preference and what your goals/tasks/workflows are.
You are using qubes now, so just experiment! You have unlimited qubes you can play with and find out what you like!
Go for it!
Oh and another thing: Qubes really starts to shine with the dispVMs and AppVMs. Start how you like to do things, but with those features you have more control and security instead of using just a couple of standalones. This is just a can not a must of course.