If you've already set up your zsh as you like it, I don't think there's a huge amount of benefit. The big benefit of fish is that it's pretty much the ideal shell, but straight out of the box, without configuration. Autocomplete just works, navigating the history is easy, it completes commands from your history, choosing a shell prompt can be as simple as opening a browser, etc. You can customise it pretty much as deeply as you need to - just like zsh - but the defaults are good enough that you've got a really nice shell out of the box.