Is there a general recommendation regarding which software package to select in the software center?
I found enough sources on the difference of Flatpack vs Snap. But what about Flatpack vs Zorin? Is the any preference by the OS? Does something run better?
Zorin apps are generally the first choice. If you need newer features of an app and the latest version, it is best to use a Flatpak (except for the Brave browser, which is always up to date) or to look on the developers site of the program if they offer a .deb package or even a repository to get the latest version.
The first Thing: .deb Packages are the native Package Format. Snaps and Flatpaks are universal but run isolated with limited Permissions. And this can cause the Case that You ahve to set up the Permissions so that a Program runs. With .deb Packages, You don't have that.
Because Zorin is an LTS Distro, the .deb Packages are older - with some Exceptions. Flatpaks and Snaps are (mostly) more up-to-date. A downside is that they come with own Dependencies which make them bigger and so need more Disk Space. When You only have limited disk Space, this could be a deciding Factor.
When You need/want an up-to-date Package, You could go to the Developer's Website and look if You can get there a .deb File or if there is offered the adding of a Repo to get the up-to-date Version. It would lead that You would get Updates through the offered Repo. So, You would have to manually update the Software.
At the End: Use what You like and what suits You. I personally have made bad Experiences with Snaps. So, I use .deb and Flatpak.
Adding on Discord, Steam as also updated by the maintainer in the same way, so the Flatpak won't bring you a later version of those, either.