They'll do what when they're actually in the situation?

They will take another look at the available options and pick what they think is the best option. This may be different from the one they proclaimed they would take.

How do they determine which is best?

The same way you determine anything in your life. You make tons of decisions every day based on the situation.

but what are the criteria by which I make the decision?

Np, I actually have values. It's difficult to believe, most of the people around you apparently have none. First off, my own organs are spare parts for those in my family who are younger than I am... children, nieces, nephews, etc. They're never to donate to anyone in a generation older than them (lateral's sometimes ok). And I wouldn't accept donor organs from someone not in my family. I've got other people to think about than myself, so staying alive at any cost (financial or moral) isn't a consideration at all. Not saying I wouldn't hesitate if facing the end, but I've thought about this alot over the years and I've already made the choice.