The OP makes reference to a monthly pension - That's what made me think that an Annuity was what he was currently in consideration of - Perhaps it isn't but that was my first port of call as I am not all that up on where to invest funds. Not really my expertise
Not always the best option.
If you expect a long and healthy life, long into retirement then the larger lump sum isn't always the best option - If you intend to get 20 years or so out of the monthly Annuity then it could be beneficial to take the larger Annuity
I know you can't anticipate...