There's a whole lot of comfort in a guaranteed income stream. That comfort can't be quantified.
But to see some numeric results, run http://capn-bill.com/fire
and reduce your expenses by the amount of the pension in the lower section, starting in year 1 -- uncheck the inflation box.
You'll get two figures -- the percent survival and the mean ending portfolio.
Then run it again, but set the withdrawal reduction in the lower part to zero, and increase the portfolio by the amount of the lump sum.
The differences in survival and average ending portfolio balance might help you.