It's a good question actually. More people should put thought into their dependencies!

The latest non-beta version of both the editor and runtimes is generally the most stable and has the most fixes. Right when we move from beta to non-beta there may be some issues that we have to scramble to get fixed ASAP. We try our best to minimize that, but sometimes problems aren't apparent until after that move. Once a non-beta version has been around for a while, as 3.8 has, then you can be sure it is stable. 3.8 is our most stable release ever, especially since we recently fixed a few "mystery" bugs that we suffered for a long time!

The latest beta version, when available, has new functionality but you may not want to use it for production as it may have some features still under development, issues we haven't gotten to fix yet, and not all runtimes may have support yet.

Sorry for the late reply, the easterbunny must have distracted me.

There is not much that speaks against using the latest stable installment of Unity 2019.3 if you are interested in the URP 2D lighting feature set.

There is one known issue left with Unity 2019.3 that we did not get to fix yet (but hopefully will get to soon), it was a new feature introduced in Unity 2019.3 (scene reload modes):
[unity] Support new Unity 2019 Enter Play Mode · #1621

In case you should discover any problems with officially supported Unity versions, please always report right away, we will be happy to fix them.

Regarding spine-unity, as usual the latest 3.8 package on the download page will be the recommended one:
Spine Unity Download