Use Details Only with .xlsx when the file will be sorted, filtered, pivoted or loaded into another system; choose Formatted Report only when someone will read the file as it is. Switch to Details Only the moment a person says “I need to sort by close date”, because the formatted file stores dates as text.
We exported the same pipeline report both ways in our Winter ’27 org and opened every cell. Below is the exact click path to export Salesforce report to Excel, what each file really contains, and the two defaults in the Export dialog you should change before you click Export.