I'm curious about your personal experiences with physical pain. What is the most painful thing you've experienced and what was the duration? I'm sympathetic to your preference in the abstract, I just think you might be surprised at how little pain you're actually willing to endure once it's happening (not a slight against you, I think people in general overestimate what degree of physical pain they can handle as a function of the stakes involved, based largely on anecdotal and second hand experience from my time in the military). At the risk of being overly morbid, I have high confidence (>95%) that I could have you begging for death inside of an hour if that were my goal (don't worry, it's certainly not). An unfriendly AI capable of keeping you alive for eternity just to torture you would be capable of making you experience worse pain than anyone ever has in the history of our species so far. I believe you that you might sign a piece of paper to pre-commit to an eternity of torture vice simple death. I just think you'd be very very upset about that decision. Probably less than 5 minutes into it.

I'm not sure I'd call it controversial, but I have the opposite preference myself. Come to think of it, from my point of view, the fairly commonly-pushed myth of control-freak gods (insert &hellfire_preacher) looks rather similar to being tortured by an uFAI, and makes simple nonexistence look like an attractive alternative.