If a site does not have a requirement to create FGDC metadata and is new to metadata, create ISO metadata. The ISO editor is easy to use and was designed especially for beginners.
If a site does have a requirement to create FGDC metadata, or wants to create detailed metadata, create FGDC metadata. The FGDC editor provides complete support for the FGDC standard. The ISO metadata editor only supports the core elements in the ISO 19115 standard.
If a site is currently creating FGDC metadata, continue doing so.
ISO 19115 has been finalized, but it remains a content standard. ArcCatalog stores ISO metadata in an ESRI-defined XML format. When ISO 19139 is finalized, existing ISO metadata will have to be migrated to the new XML format specified by that standard. Also, the FGDC will adopt ISO 19139 and existing FGDC metadata will also have to be migrated to the new XML format. Therefore, there is not a compelling reason to switch from FGDC to ISO metadata at this time.