These are essentially two different types of deployment model at Databricks. Single-tenant is the legacy deployment architecture that is being phased out with E2 being a name for the replacement.

There were several limitations and constraints that E2 helps solve for and makes both governance and customization easier and broader. With E2 workspaces, you can create and manage multiple workspaces associated with a single Databricks account (through an API), deploy into custom VPCs, and more. There are even improvements with update rollouts and maintenance windows that will be coming in the future.

To get more details on this or to see how this can fit in with your governance strategy, I'd recommend reaching out to a Databricks representative since there are a lot more improvements than the few I've mentioned here.