"Clearly the better choice" is stating your conclusion rather than making an argument for it.

I assumed that the rest of what I wrote made it clear why I thought it was clearly the better choice.

There's an obvious reason for discounting the preferences of causally unconnected entities: if they really are causally unconnected, that means that they can't find out about our decisions

If that was the reason then people would feel the same about causally connected entities who can't find out about our decisions. But they don't. People generally consider it bad to spread rumors about people, even if they never find out. We also consider it immoral to ruin the reputation of dead people, even though we can't find out.

I think a better explanation for this intuition is simply that we have a bedrock moral principle to discount dissatisfied preferences unless they are about a person's own life. Parfit argues similarly here.

This principle also explains other intuitive reactions people have. For instance, in this problem given by Stephen Landsburg, people tend to think the rape victim has been harmed, but that McCrankypants and McMustardseed haven't been. This can be explained if we consider that the preference the victim had was about her life, whereas the preference of the other two wasn't.

Just as we discount preference violations on a personal level that aren't about someone's own life, so we can discount the existence of distant populations that do not impact the one we are a part of.

and that the extent to which their preferences are satisfied isn't therefore affected by anything that we do.

Just because someone never discovers their preference isn't satisfied, doesn't make it any less unsatisfied. Preferences are about desiring one world state over another, not about perception. If someone makes the world different then the way you want it to be then your preference is unsatisfied, even if you never find out.

Of course, as I said before, if said preference is not about one's own life in some way we can probably discount it.

It's not like the mind that we're seeking to recreate already exists within our part of the universe and has a preference for being (re-)created, while a novel mind that also has a preference for being (re-)created exists in some other part of the universe.

Yes it does, if you think four-dimensionally. The mind we're seeking to recreate exists in our universe's past, whereas the novel mind does not.

People sometimes take actions because a dead friend or relative would have wanted them to. We also take action to satisfy the preferences of people who are certain to exist in the future. This indicates that we do indeed continue to value preferences that aren't in existence at this very moment.