Today, the main difference between Polisy and eisy are the hardware capabilities. On the software side, both are similar.
However, one big difference is that some of the upcoming features will only be available on eisy:
The matter-server, which allows to expose IoX devices to matter & homekit.
The new eisy-ui service
The new eisy-ui service will provide a unified access to IoX, PG3, and some of the portal features as well. It will also serve the new web UI which will allow you to have a web-based customizable dashboard-like UI. Eventually it will completely replace the java based admin console.
Polisy will still benefits from updates to IoX and PG3, but new features like the matter-servert and eisy-ui service will only be available on eisy as it needs that extra power thar eisy provides.