DISCLAIMER: the following is not necessarily my own opinions or beliefs, but rather done more in the spirit of steelmaning:

There seems to be a number of signs that the deciding factor might be the ability to form long term memories, especially if we go into very near mode.

• It seems that if we extrapolate volition for an individual that is made to suffer with or without memory blocking in various sequences, and allowing it to chose tradeofs, it'll repeatedly observe clicking a button labelled "suffer horrific torture with suppressed memory" followed blacking out, and clicking a button labelled "suffer average torture with functioning memory" followed by being tortured. It'd thus learn to value experiences without memory much less.

• If I remember correctly some anaesthetics used for surgery basically paralyses you and disable memory formation, and this is not seen as an outrage or horrifying, even by those that have or will be experiencing it.

• If we consider increasing the intelligence of various animals while directing them to become humanlike, then by empathic modelling it seems that those capable of forming long term memories beforehand would identify with their former selves, get angry at people who had harmed them, empathize strongly and prevent the suffering in beings similar to what they were before, etc. while for those that couldn't, the opposite of these things would be true.

• If I am given the choice to have one type of cognitive functionality disabled before being tortured, in almost all circumstances it seems the ability to form long term memories would be the best choice.