If I have a choice to save a fully grown human (let's say 30 years old, the issue becomes more moot the older the person gets) or a child (yes, even a born child), I would sooner save the adult. A dead child is a waste of 9 months to 2 or 3 years of resources. A dead adult is a waste of many more years of resources. An adult is at the pick of their productivity, the child is not and won't be for many years to come.
While I agree with your first paragraph--the human mind is what matters--it might still make sense to save the child. In today's world where people die at 80, the child will probably live more years between now and death than the adult. If you factor in the fact that people who are younger today have a slightly greater chance of becoming immortal, this consideration becomes much more important. Of course, if the child is young enough that it doesn't have a human-like mind, the adult then has a greater "right to life" (quotation marks because not everyone thinks rights are a useful heuristic).
I've thought about this some more and the point isn't who will live more years, but rather how much utility can each one produce. If it's a choice between a 4 month old child and a 30 year old adult, and we choose to save the adult, then in a year we can have another 4 month old (and not that much different), while the adult is continuing to provide utility. If we save the child, then it will be about 18 years before the child's rate of utility output can come close to the adult's, and meanwhile we lost all the utility from the adult. If I thought about this more, I could probably write a function, which will determine who you would save.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.