There was a time Nepo Systems was the only option available. Unfortunately the company adopts an extremely restrictive licencing policy. I worked fdor SugarCRM Support. My request to have a single licence to learn, demo and help customers did not succeed. Please do not take me wrong. Nepo is a great company and I am glad to praise their good work.
The Marketo Connector for Sugar
is developed and maintained by SugarCRM itself. The lead developer, John Mertic, is one of the most active members of SugarCRM's Engineering team. He wrote two reference books.
I am going to request a SugarCRM demo/test instance. I will post my findings on Marketo and SugarCRM Communities.