You really need to crank a few numbers to determine if lump sum or all at once makes sense:
What is the discount rate applied to the funds when calculating the lump sum? In some cases, the discount rate APR is much higher than can be earned on any safe investment.
Who is responsible for paying you? The full faith and credit of the state, the credit of an quasi independent agency, or a third party investment house from which the state bought the annunity?
What are the survivorship rights - does the 20 years get paid no matter what, or end when you die?
What are the tax implications of all at once vs. annuitized. If you live in a state with an income tax, but plan on moving to a state without one, the yearly payout could save 5% or more (over 10% in NY state, due to their graduated income tax)
How fast do you plan on spending it down? If you have stage IV cancer or have had three bypasses and want to spend it all before you die, you will need to take a lump sum and get busy.
How much does Mike Dillon want to sell you one of his transferrable miniguns?
+1
All excellent points.