This depend on the current configuration of your receiver system like -
> is it a multiclient system or a single client
> how much data does the current client ( to be deleted) has as compared to other clients.
> is the current client already (almost) blank
So based on above the system makes a decision on which method would be best and the decision so taken is the most optimum and should not be changed.
If at all it is required to be changed then the same should be consulted with an expert.
Please refer to chapter 5.5 on the SAP TDMS Operations Guide. Drop insert is generally faster but you cannot use it in a multi-client system unless you are able to lock the users in all clients.
If the receiver system has more than one client and you cannot lock users in other clients than you must choose array-delete, otherwise it is recommended to choose the deletion settings from activity 'Define Deletion Scenario Automatically'.