The reason religion is so powerful

[John 6IX Breezy]

That's an excellent question. If I had to make a choice, I would save the baby. It's not unlikely the elderly person would want me to save the baby too. All stories must come to an end, and I personally would rather have mine wrapped up a few pages sooner than I planned than live with the knowledge that my survival came at the cost of someone with their whole life in front of them.

I think the reason we would choose an infant over an elderly person (and you hinted at something similar) has something to do with Thomas Nagel's famous argument: Death is evil insofar as it deprives us of life. Therefore, the younger you are the more life you can be deprived of, and the more tragic the death. And abortion, I would argue, logically deprives an organism of the most life possible.

ps. I hadn't thought about your pregnant vs nonpregnant scenario before, and I think its a question that reinforces my object permanence argument, given that a pregnant woman can be visualized in a way that an embryo cannot. Therefore, I suspect that someone who would be fine with a two-month abortion on paper, would nevertheless be inclined to save a woman that is pregnant over her twin sister that is not.