Extend Report: No CSS will be available local. If report has to be show in nice GUI internet should be there. Same to the user who is opening the report. Unless they dont have internet, you cannot see the proper data
Allure: Learning curve. will have the CSS embedded in the html
Allure doesn't allow to have single html file that you could email to someone , it requires you to spawn up a jetty local server to view the report. Where as extentreport allow you to have a single html file.
This is the only difference between the two, other than that the report generation is same in both the reports , you have to use helper methods ...