Hi,
The validation put in save event handler will only be applicable at the Web UI level. If a user tries to make the same changes using transactions in SAP GUI, the validation will not come into effect since its written only for save event.
But a validation placed in ORDER_SAVE badi will be applicable for both Web UI changes and SAP GUI changes. It is always a good practice to put the custom logic in the exits/BADIs provided to us rather than enhancing the BSP component and placing it in event handlers.
Hope this helps,
~Athreya