AFAICT, the argument has nothing to do with the problem at all, and everything to do with defending "your" side.
My initial response was "halfer," the naively obvious answer. Then, knowing that these problems always have a trick, I examined the precise phrasing of the question more closely, and "thirder" is clearly correct. That's the -point- of the problem, and what makes it interesting - it's designed to make you come to the wrong conclusion using naive logic, for the pure purpose of showing that the naive logic is, well, naive. We wouldn't be discussing the problem if it didn't have that property - if the naive solution wasn't wrong, it would be a completely uninteresting problem.
Spend some time guessing the teacher's password, people, before you marry your answer, and then proceed to spend hours trying to invent a novel reason why your answer must be the correct one. The problem exists -because- it defies your expectations. Instead of trying to justify your expectations, take a look at what the problem is trying to teach you, because that is what it was designed to do.
TLDR? The Sleeping Beauty problem was designed to impart a lesson about naive logic. Quit fighting the lesson.
Care to elaborate?
You just woke up. You don't know if the coin was head or tails, and you have no further information. You knew it was 50-50 before going to sleep. No new information, no new answer. I don't see what the "twist" is. Monty Hall, there's another information input - the door the host opens never has the prize behind it.
Or, another perspective : a perfect erasure of someone's memories and restoration of their body to the pre-event state is exactly the same as if the event in question never occurred. So delete the 1 million from consideration. It's just 1 interview post waking. Heads or Tails?
A friend referred me to another paper on the Sleeping Beauty problem. It comes down on the side of the halfers.
I didn't have the patience to finish it, because I think SB is a pointless argument about what "belief" means. If, instead of asking Sleeping Beauty about her "subjective probability", you asked her to place a bet, or take some action, everyone could agree what the best answer was. That it perplexes people is a sign that they're talking non-sense, using words without agreeing on their meanings.
But, we can make it more obvious what the argument is about by using a trick that works with the Monty Hall problem: Add more doors. By doors I mean days.
The Monty Hall Sleeping Beauty Problem is then:
The halfer position implies that she should still say 1/2 in this scenario.
Does stating it this way make it clearer what the argument is about?