If you are comparing these two, the first question is whether you want to run the software yourself.
Hosting and control
Postiz is open source, so technical teams can self-host it and change the code. It also offers hosted plans for those who do not want to run it.
Zedgad is hosted only. You do not manage servers, updates, or platform API changes; you connect accounts and publish.
Pricing
Postiz hosted plans are priced by number of channels, from 5 channels at $29 a month up to 100 channels at $99 a month, with API and webhooks included.
Zedgad is priced by number of projects, from $9 to $99 a month, with unlimited posts. X publishing is an add-on.
Which one to choose
Choose Postiz if you want to self-host, modify the source, or need its wider set of supported networks.
Choose zedgad if you want a hosted tool with a lower entry price, a WordPress plugin, and no infrastructure to maintain.
Key takeaway
Postiz suits teams who want to self-host; zedgad suits teams who want nothing to maintain.