Azure Blockchain Service is a managed service to host blockchain network. Creating a instance on managed service would mean you would not have to worry about keeping the instance up and running and updated to the latest at all time, the service providers will take care of the maintenance and patching. With ABS we currently support Quorum ledger. Please refer to the documentation here https://learn.microsoft.com/en-us/azure/blockchain/service/overview
Azure Blockchain Workbench is a template offering that is more to help customers to get started with their blockchain PoC. The template takes care of setting up the end-to-end blockchain application including the different services such as web client, Service bus, event grid etc along with the blockchain node itself. While deploying a Workbench instance user has an option to choose to deploy the blockchain node by using an Ethereum template of ABS itself in which case an ABS blockchain member will be created. Please refer to the documentation here https://learn.microsoft.com/en-us/azure/blockchain/workbench/overview
Source: MSDN