Both programs transfer the exact amount necessary to cover your check or withdrawal and maintain your checking account balance at zero, but the Overdraft Line of Credit is a Loan, which you pay back with interest.  The Overdraft from Savings transfers your own funds from your savings for a fee instead of interest. Neither
has a cost if it is never used. Since the Overdraft Line of Credit is a loan, you must apply and be approved for it; Overdraft Protection from Savings is available on request.