If all you need is basic splits, sessions, and some simple templates/layouts (and like the convenience of knowing that tmux is widely available, and often installed by default) then you're fine to stay on tmux.
Zellij can do things like floating windows, contextual keybinding guidance (helps learn everything that can be done), and a more complex layout schema. You can disable all the UI eye-candy and switch to tmux-style bindings too.
It's worth trying out. I use both so that I can still function on systems without it.
If you are comparing raw features, there is probably little to differentiate vs a legacy tool. However, the out of the box experience is fantastic, and the author has clearly put a lot of effort in their take without being locked into legacy decisions.
If you watch some of the screencasts and are not impressed, there is nothing more I can do to pitch the idea. My only complaint is that I feel like I am only ever scratching the surface of what the tool can offer.
Zellij can do things like floating windows, contextual keybinding guidance (helps learn everything that can be done), and a more complex layout schema. You can disable all the UI eye-candy and switch to tmux-style bindings too.
It's worth trying out. I use both so that I can still function on systems without it.