In the spirit of keeping things simple...
Can anyone provide a simple practical comparison, or specific stumbling blocks I will need to know, so that I can start working towards being ready for the new shell when Catalina is released?
If you're thinking using the new default shell consider:
- If you want to give Zsh a whirl and feel some of the differences without changing shell settings on your machine you might try Powerline10k in a Docker container and see if it is your cup of tea.
- If you don't need all the bells and whistles and use Bash for just basic scripts it's rather easy to set your shell as others here have explained. And if you decide you want to use features in Bash 5 it's a fairly trivial upgrade for macOS.
- If you want to improve the portability of your scripts so they're more likely to function as expected in both shells, test them for POSIX-compliance and remove any "bashisms". I've used ShellCheck for this and it works quite well for less complicated scripts.
While no particular path is clear these three approaches should give you enough confidence to make an informed decision without over engineering the problem or solution space.