The difference between 5.3 and 6 are the Qt framework they use, the functionality is the same. 6 could work better for you if you work on a Linux with Wayland and you had trouble with wayland before, otherwise 5.3 is recommended (in my opinion). 6 still has a few bugs that 5.3 doesn’t have and are related to Qt. So if you don’t absolutely need 6 I would stay away from it for now. I used the Krita 6 beta for a while and it has some clunkyness with the UI (especially with the new text tool).
I personally recommend 5.3 especially if you care about things working consistently. There are still tons of minor bugs, issues and crashes in 6.0, while 5.3 should have a lot less of those.