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.
The 4 month old will produce 80 years of utility if saved, for 18 years of investment: if this is a bad bargain, reproduction always is. The adult, if saved, will produce 50 years of utility for 0 further investment. This means a higher rate of return on the adult, but a higher total future "utility revenue" from the child. Depending on who's making the investment and whether that matters to you, the answer could be either way. But now that I've written it out like this, I can see your point: the adult does have a higher rate of return.
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.