Both zsh and fish offer the same kinds of improvements over bash. The headline feature of these two is the auto completion user experience. They complete as you type, with fish for example showing suggestions in a muted colour ahead of your cursor which you can complete word by word or accept completely.

zsh is a much older project and carries with it a lot of baggage in terms of configuration. fish was designed later and straight out of the box.

Intelligent auto complete is extremely useful if you are constrained with your input either by disability or by using a suboptimal device. Switching to fish really helped for ssh-ing to hosts from my iPhone.