Moderator API: If you have an existing implementation for reviewing and/or acting on flagged content, and would like to just use the APIs for scanning speed, coverage, and scale, use the Moderator API to scan your content and process the results at your end.
Review Tool: If you are looking for an end-to-end, configurable solution for integrating and scanning your content, reviewing flagged items by a human review team, while complying with your escalation workflows and content policies, the review tool is a great new product to try out. We will be constantly adding support for new content types, adding more extensibility options, and integrating custom classifiers for even better fit with your business and users.
Both: You either have an existing Moderator API subscription that you would like to use with the review tool, or you could be using the review tool with one content type (images) while using your own systems for another content type (text). In any or all combinations of these scenarios, it's completely fine to use both the review tool and the API. The reports and dashboard will only track content that flows through the review tool.