When to use Agency Sharing:
If you are part of an agency and want to make a template accessible to anyone within your agency, you should use agency sharing. This will give all agents within the agency access to your template.
When to use Community Sharing:
If you are not part of an agency (or if you are, but you want to share more broadly with other Tern users that are not only part of your agency) and have a group of advisors you would like to share content with, you should set up a Community.
This allows you to decide how sharing in your Community will work, with options ranging from allowing all members to share, to restricting sharing to only owners and admins. Communities are suitable for developing specialty content and sharing it more easily with Tern users. They are not limited to one agency.