I think the feature to compare the difference in grants (object level permissions) in one schema with other schema is not available
That seems to be a reasonable conclusion since the Database Diff wizard focuses on owned objects and looks like...
and then with Step 3 allowing you to further filter the set of objects to compare.
If you want to stay within the SQL Developer tool and avoid web searches looking for scripts to do this, I suppose you would go to View > Reports, then drill down in Data Dictionary Reports > Security > Grants and Privileges > Object Grants for the specific users/schemas. Of course you can save this report as a User Defined report, customizing it as needed, then compare the output with your favorite diff utility.
Thanks for your suggestion