However my personal shell preference is zsh. When I write scripts I do so using bash. The two shells are 99% similar on a day to day basis, but I prefer zsh for a user interface. So I use one for day to day and the other for scripting.

I used to do the same, Bash as the default script interpreter and ZSH as my terminal interactive shell. Worked well. I also had Dash as the /usr/bin/sh installed. But once I installed a new OS, I did not want to set it up immediately and wanted to do it later. Then I realized that I do not need to and frankly didn't notice any difference. Besides ZSH was a bit slow here and there with the kind of setup I had. So since then I stayed on Bash only again and don't miss most of the stuff (but ZSH had some nice features).

I wouldn't have even mentioned Fish here, its too different. If you going to learn a scripting language that is this different, then there are even more shells out there. I would stick to Bash (or ZSH for that matter). Especially if its the first shell language you get into.