The choice would depend on your framework, your process and the ways you want the result to appear in cycles.
If you are using Cucumber, Robot or Postman, AIO Tests gives step wise results and hence an automated case may be different from manual if many manual cases make up one automated case. However, if you are using the APIs or frameworks like TestNG, JUnit, NUnit, then manual cases and automated cases need not be separate since only case level results get updated from automation.
Based on your process, the cases if you write are already end to end, then you need not create separate automated cases and can simply mark the automation status of the manual case.
You can schedule a meeting with AIO Tests Support or book the office hours to discuss this further from the Help icon in the app.
OK, How Aio Calculates Time Saved.
Every case has a field called Estimated effort. The estimated time comes from this case field. The execution time can be tracked using timer for manual executions and comes from framework specific reports for automated executions.