Looking at their descriptions in Synaptic, they seem identical. Does it make any difference which I install?
They are identical. The ZorinGroup has the Zorin Repository and the Ubuntu Main Universe repository. Packages contained as a Zorin Repo get the Zorin name.
as Aravisian said they are identical. I would personally just install the Zorin one as anything to do with the operating system team itself instead of what it's based from can help further down the line if you need support.
Plus it's just good practice to use official packages from the operating system you're using [ZORIN OS] even if there is a package from what it is based on [UBUNTU] as the package from the Zorin team has been designed to work with the system you're running.
I hope this explanation wasn't confusing for you
Thanks @Aravisian & @justinc . Back in December, I remember a link saying one of the first things to do with a new Zorin installation was to install Ubuntu Restricted Extras. When I searched for 'restricted' in Synaptic this time, I saw there were also Zorin Restricted Extras. I've installed the Zorin ones this time.