These modules achieve the same thing - allow you to choose which display mode to use for a referenced entity. The differences are in how they are configured.
Entity Display Mode provides a field you add to the entity you want to add the switcher to.
Field View Mode is configured through the site's Display mode settings, which I think is hard to find. Once you configure it here, it adds a field to the entity you selected. It doesn't allow default as a selectable option, and doesn't let you edit the field label.
Entity Reference Display is a field you add to the referring entity. This means that to apply it to existing entities, you'll have to replace all the reference fields.
There is also this module, which has been in dev for a few years:
https://www.drupal.org/project/entityreference_view_mode
And also:
https://www.drupal.org/project/view_entity_ref_field_formatter