I find Fish to be simpler to use and more powerful by default. You can probably have a zsh or maybe even bash setup which does the same fish does, but I don't want to do that.
(edit: this was a reply to the post below.. not sure how I messed that up.. gah.. such a noob)
Another thing is writing scripts and completions functions with fish, it's a way better experience than zsh. Writing completion stuff using zsh is really painful.
The preview with the autocompletion is really cool, but maybe you can have that with zsh too.
And then there's the speed, it's totally not objective but IIRC my prezto setup was quite a bit slower.
In the end I just really appreciate that I don't have to spend time customizing my shell: now I install fish on my VMs or servers, and everything is like I want it without me doing anything.
If you're happily using prezto you may not have much to gain from using fish, and you may not even like it since there's stuff from prezto you won't find out of the box.