In annihilation the sum of pain and pleasure is zero, unlike how in eternal suffering pain is more than the pleasure so the sum total is less and therefore worse. Both pain and pleasure could be seen as on the same scale , like how negative and positive numbers are on a number line. The common unit would be utility of the person. Anihilation can be compared to an very painful afterlife because it would have an effect on the total happiness of the dead person, like how zero is on the number line. They can be compared because a person could prefer one to the other naturally. The state of deep sleep gives us someone idea of what annihilation might be like, and although we are always conscious we could experience stopping some of our senses with things like blindfolds. This would allow annihilation to be compared to eternal suffering. If we wish for everyone to be as happy as possible than
annihilation would be perhaps a better option than eternal suffering. I do not believe that eternal suffering would exist after death, instead I think some random mind in the future or past or annihilation would be experienced.