So in this case, you would need to find the present value (PV) of the monthly payments. With the information given, you would have a PV= 195,413.08, which is less than the lump sum payment. In this case, you would take the 1 time payment.
Another way to look at this is to calculate the future value (FV) of both payouts. For the lump sum payment, you would assume the same interest rate (6%) and at the end of the same 20 years period, your investment would be worth 662,040.90 while the monthly payment option would be worth 646,857.25