The Geniesoft website has the latest update for Overture listed as "3.6". The Garritan website has an update listed as "3.5" that indicates it is optimized for GPO. Which one do we use for best integration of GPO/GPO Studio/Overture ?
You want the latest one on Geniesoft's website. Geniesoft is responsible for Overture SE so they have the latest and greatest info. A lot has been changed (made better) since the 3.5 update on the Garritan website.