The notion of Essential vs Advanced Payment Solutions is more a marketing difference between 2 separate groups of payment gateway services. The offered services are essentially the same.
CyberSource Payment Gateway (Advanced):
This is the Advanced Solution offered to Medium and Large size businesses. CyberSource supports approximately 20,000 such customers.
The Zen Cart (ZC) Payment Module supports the Advanced Solution. The ZC CyberSource Payment Module uses the Silent Order POST (SOP) Connection Method to link to the CyberSource server.
CyberSource used to accept customers no matter the business size. This policy may have changed lately.
Authorize.Net Payment Gateway (sometimes refer to as Essential):
Authorize.Net offers similar Payment Gateway services to small business (<$3M online). Authorize.Net supports approximately 175,000 such small business customers.
Authorize.Net is wholly owned by CyberSource following the 2007 acquisition.
The Authorize.Net Payment Module is included in the ZC package when you download it from the ZC portal. Getting the Authorize.Net Payment Module to work once ZC is installed on your host server is a rather easy task. Only a few parameters need to be set and you are ready to go. The Advanced Integration Method (AIM) is the recommended version of the module.The Authorize.Net Payment Module is fully supported by the ZC Development Team.
Please compare the rates between the 2 solutions. You may be better off selecting the Authorize.Net Payment Module since it is fully supported by the ZC Development Team. Also, please check with your merchant account provider (bank) concerning other possible constraints.