Depends on information. If people retain memories, so that each person-moment follows from a previous one, then knowing only that I suddenly find myself in a room means I'm probably in room A. If people are memory-wiped at some interval, then this increases the probability I should assign to being in room B - probability of being in a specific room, given that your state of information is that you suddenly find yourself in a room, is proportional to the number of times "I have suddenly found myself in a room" is somebody's state of information.

The above is in fact true. So here's a fun puzzler for you: why is the following false?

"If you tell me the exact time, then my room must more likely be B, because there are 1000 times more people in room B at that time. Since this holds for all times you could tell me, it is always true that my room is probably B, so I'm probably in room B."

Hint: Assuming that room B residents "live" 1,000,000 times longer than room A residents, how does their probability of being in room B look throughout their life, assuming they retain their memories?

Here's a stab: If I understand you correctly, then every observer's experience is indistinguishable from every other's, so my credence in the proposition "I'm in room A" is 0.999 and my decision policy is "Bet that I'm in room A." If 100 trillion + 100 billion people choose room B, then 100 trillion will lose and 100 billion will win. If 100 trillion + 100 billion people choose room A, then 100 billion will lose and 100 trillion will win.

I don't understand how it could happen: do you mean that time in the room B is million times slower?

Recommending an edit: The thousand instances of people in their respective instances of B rooms at time t are distinct situations, so it's confusing to say they're all in the same room. Say they're in red rooms while the others are in blue rooms, or something. A kind of room rather than a particular room.

It's sort of important to stress this because some patternists will genuinely try to erase the distinctions between the different rooms, you say, "you're talking about them like they're distinct, even if they encode exactly the same pattern you know they're conceptually distinct" but they just pretend they don't understand this and it's very annoying.

Phil of FB: (A more concrete example: 10,000 people are traveling to Mars. 1,000 board a large slow shuttle that takes a single trip to Mars between t1 and t3. Meanwhile, a really fast smaller shuttle takes 10 people at a time to Mars (going back and forth 900 times) during this same period. At time t3, all 10,000 people have safely arrived on Mars. If asked, at t3, whether one took the large slow shuttle or the fast small shuttle, one should say the latter. (Right?) But this is the opposite answer, I believe, that one should give if in the middle of the journey, at time t2, one is aroused from one's hibernation (let's say) and asked whether they are at that very moment on the slow or fast shuttle. Thus, it seems to matter whether the relevant event is ongoing or over. But I’m not exactly clear about why.)

My reply: Imagine there is a random person BOB. If Bob asked before flight to Mars, he will said that he will most likely fly small and quick spaceship. But if we ask a random person during the flight (And if it he is Bob - which is important point here) - than Bob is most likely on a large space plane. But the difference in both situation is that we must add probability that random person will be Bob. And this probability is rather small and will exactly compensate. The fact which is not represented is that there is third group of all travellers, which are already on Mars or wait start on earth, and when I am told that in the moment T3 I still flying, I get information, that I am not one of 8990 "waiters" and update my probabilities accordingly.

In Bostrom's dissertation he says it's not clear if number of observers or the number of observer-moments is the appropriate reference class for anthropic reasoning.

I don't see how you are jumping to the fourth disjunct though. Like, maybe they run lots of simulations which are very short? But surely they would run enough to outweigh humanity's real history whichever way you measure it. Assuming they have posthuman levels of computational power.

How many seconds have you been in the room?

Let's say the time between t1 and t2 is 1 trillion seconds. Let us further assume that all people go through the rooms in the same amount of time (thus people spend 1 second each in room A, and 1 million seconds each in room B).

100 trillion of the 100.1 trillion observer moments between 0 and 1 seconds in a room occur in room A. All of the observer moments past 1 second occur in room B (this is somewhat flawed in that it is possible that the observers don't all spend the same amount of time in a given room, but even in the case where 100 million people stay in room A for 1 million seconds each, and the rest spend zero time, an observer who's been in a room for 1 million seconds is still overwhelmingly likely to be in room B. So basically the longer you've been in the room, the more probably you should consider it that you're in room B).

If an observer doesn't know how long they've been in a given room, I'm not sure how meaningful it is to call them "an" observer.