Assuming that there are 3^^^3 distinct individuals in existence, I think the answer is pretty obvious- pick the torture. However, the fact that we cannot possibly hope to visualize so many individuals it's a pointlessly large number. In fact, I would go so low as one quadrillion human beings with dust specks in their eyes outweighs one individual's 50 years of torture. Consider- one quadrillion seconds of minute but noticeable pain versus a scant fifty years of tortured hell. One quadrillion seconds is about 31,709,792 years. Let's just go with 32 million years. Then factor in the magnitudes- torture is far worse than dust specks- 50 years versus 32 million good enough odds for you?
However, that being said, the question is yet another installment of lifeboat ethics, and has little bearing on the real world. If we are ever forced to make such a decision, that's one thing, but in the meantime let's work through systemic issues that might lead to such a situation instead.