By instincts, everyone would try to save their own lives first. Normally, saving the other one means you think it'd be better if he or she lived instead of you, which means you think your life is not worth. Those who want to leave their families, please don't be afraid to say it. It's normal. And it's not selfish because you can't do much for them. If only one will survive, it's your decision, and, as I said, the average person would prefer his or her own life.
I'm not going to answer, though. I'm not thinking about what I'd do in that situation. I'll just let it happen when I get there.