Here are the main difference between each other:
Anyconnect Essentials Anyconnect Premium
Does not support CSD Does support CSD
Does not support Clientless SSL Does support Clientless SSL
Only supports AnyConnect client connections Supports all flavours of SSL VPN, including:clientless SSL VPN, AnyConnect client VPN
The OS of the ASA has a software switch in the VPN config that only allows for the ASA to be in one scheme or the other at any one time so you cannot have both and Essentials and Premium license active at the same time.
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC