“Optimists and pessimists differ only on the date of the end of the world.”

As quoted in Serving "60 Years to Life", Newsweek Europe (12 December 2005)

Humor in Psychotherapy (2007)

Source: Reflections

Variant: Both optimists and pessimists contribute to society. The optimist invents the aeroplane, the pessimist the parachute.

Quoted in Women Know Everything!: 3,241 Quips, Quotes, and Brilliant Remarks By Karen Weekes, p. 41

This is derived from a statement of James Branch Cabell, in The Silver Stallion (1926) : The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.
Misattributed
Variant: The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.

Introduction, p. 1.
The Best of All Possible Worlds (2006)

Epilogue, p. 242
Out of My Life and Thought : An Autobiography (1933)

Dear Me (1977), p. 167