Ideally there would be only one version, the XML + XSLT version. The main advantages of this format over HTML are:
The problem with client side XSLT is that support for it on the "file:" protocol is problematic in many browsers.
You can use either version anywhere, but the HTML version would be best on the "file:" protocol while the XML version would be best over http.
Note: to support client side XSLT on the "file:" protocol the Google Chrome browser needs to be started with the command line flag --allow-file-access-from-files