This is a really interesting issue which I suspect will only get more important over time. I largely agree with Xachariah, but I see a greater dependency on personal preference.

Another way of looking at the problem is to consider individual preferences. Imagine a radical sustainable future where everyone gets to choose between an extended life with no children or a normal life with 1 child (or 2 per couple). I'd be really interested in polls on that choice. Personally I'd choose extension over children. I also suspect that polls may reveal a significant gender gap on this issue, but that's just speculation.

I have a suspicion that linear aggregation of individual utilities over all possible people is perhaps not the best global decision process. There probably should be a discount factor where we discount other potential beings in proportion to their dissimilarity to existing people. We should count existing people's preferences somewhat higher than future descendants who in turn we weight higher than distant descendants and other more remote possible people.