I am basing my reasoning on the probable preferences of those involved, so my answer would depend on the feelings of the people to being dust specked/tortured.

I'm not entirely clear what exactly you are asking with number 1: are you just asking 1.6 seconds of torture vs. 3^^^3/ 1 billion dust specks? If so, I'm essentially indifferent, it seems like both are fairly inconsequential as long as the torture only causes pain for the 1.6 seconds.

For number 2, a billion dust specks would probably get to be fairly noticeable in succession, so I'd prefer to get 1.6 seconds of torture over with, because that isn't really enough time for it actually to really be torturous (depending on what exactly that torture was) rather than deal with a constant annoyance.