Wait, does the original question simplify to:
"[There exists 3^^^3 people] AND [of the set of all people there exists one that is tortured for 50 years OR of the set of all people, all get a mote of dust in the eye; which would you prefer]"?
Because that would be quite different to:
"[of the set of all people there exists one person who will be tortured for 50 years] OR [there exists 3^^^3 people AND each of them gets a mote of dust in the eye]; which would you prefer?"
I answered the latter.
The point of the question was to ask us to judge between the disutility of many people dust specked and a single person tortured, not to place a value on whether 3^^^3 existences is itself a bad or a good thing.
So, kinda of the former interpretation, except that the "3^^^3 people" part is merely the setting that enables the question, not really the point of the question...
EDIT: Btw, since I'm an anti-specker, I tried to calculate an upper bound once, for number of specks... It ended up being about 1.2 * 10^20 dust specks
"What's the worst that can happen?" goes the optimistic saying. It's probably a bad question to ask anyone with a creative imagination. Let's consider the problem on an individual level: it's not really the worst that can happen, but would nonetheless be fairly bad, if you were horribly tortured for a number of years. This is one of the worse things that can realistically happen to one person in today's world.
What's the least bad, bad thing that can happen? Well, suppose a dust speck floated into your eye and irritated it just a little, for a fraction of a second, barely enough to make you notice before you blink and wipe away the dust speck.
For our next ingredient, we need a large number. Let's use 3^^^3, written in Knuth's up-arrow notation:
3^^^3 is an exponential tower of 3s which is 7,625,597,484,987 layers tall. You start with 1; raise 3 to the power of 1 to get 3; raise 3 to the power of 3 to get 27; raise 3 to the power of 27 to get 7625597484987; raise 3 to the power of 7625597484987 to get a number much larger than the number of atoms in the universe, but which could still be written down in base 10, on 100 square kilometers of paper; then raise 3 to that power; and continue until you've exponentiated 7625597484987 times. That's 3^^^3. It's the smallest simple inconceivably huge number I know.
Now here's the moral dilemma. If neither event is going to happen to you personally, but you still had to choose one or the other:
Would you prefer that one person be horribly tortured for fifty years without hope or rest, or that 3^^^3 people get dust specks in their eyes?
I think the answer is obvious. How about you?