It’s been a while since I’ve tried zsh, to be honest, but from what I can remember and what I’ve seen online, is that fish a little bit more “modern” and has sensible defaults out of the box. While you can probably achieve the same with zsh (and probably bash as well, with lots of tweaking), fish is just easier to get up and running as supposed to having zsh and installing some sort of plugin manager such as “oh my zsh” and researching and installing a bunch of things yourself.

My experience with zsh wasn’t amazing, maybe because it may have required a bit of tweaking and I didn’t feel like it at the time. For fish, I just changed the shell, researched a little about available plugins, and decided I wantef fzf and tide, and that’s basically it.

I don’t know if either fish or zsh is more customizable, but I’m happy with it. Bottom line appears to be, zsh has been around very long, like bash, but fish is much newer, therefore feels more “modern”, whatever that may mean. The flip-side of that is that zsh is a bit more like POSIX-compliant, even though I don’t think it really is. So zsh is more like bash than fish is, while offering more features than bash.

Try out both, if you have the time. :) If you don’t have the time, go with fish.