Auto_Open or Workbook_Open? (Auto runnig a macro on file open)

Keep it simple:

Both Auto_Open and Workbook_Open are for running some code on the loading of a workbook. But they have some minor differences.

Auto_Open

Workbook_Open

Both macros will be disabled when you open a workbook while holding the Shift key on your keyboard.

 

Therefore, a most critical criteria to consider when one to be used is : do you want the macro to be fired when the workbook is opened with a macro.
(e.g. Workbooks.Open <file_path> )