The largest difference between the two is that a document policy in one frame does not necessarily affect its embedded content; the parent can switch off synchronous XHR for itself, while allowing its children to make the same decision on their own. Document Policy does define a mode by which child frames can be required to disable sync-xhr, but this requires an explicit opt in by the embedded document, and is not simply imposed by the embedder, as it is with Feature Policy.
Following the discussion in https://github.com/w3c/webappsec-permissions-policy/issues/410, it seems that there is interest in replacing the existing integration with Feature Policy with a similar Document Policy integration.
The largest difference between the two is that a document policy in one frame does not necessarily affect its embedded content; the parent can switch off synchronous XHR for itself, while allowing its children to make the same decision on their own. Document Policy does define a mode by which child frames can be required to disable sync-xhr, but this requires an explicit opt in by the embedded document, and is not simply imposed by the embedder, as it is with Feature Policy.