Cedric and Bertrand want to see a movie. Bertrand wants to see Muscled Duded Blow Stuff Up. Cedric wants to see Quiet Remembrances: Time as Allegory. There's also Middlebrow Space Fantasy. They are rational but not selfish - they care about the other's happiness as much as their own. What should they see?
People want different things, and the different possible disagreement resolving mechanisms include the different varieties of utilitarianism.
In this view, the fundamental issue is whether you want the new entity to be directly counted in the disagreement resolving mechanism. If the new entity is ignored (except for impact on the utility functions of pre-existing entities, including moral viewpoints if preference utility is used*), then there's no need to be concerned with average v. total utilitarianism.
A general policy of always including the new entity in the disagreement resolving mechanism would be extremely dangerous (utility monsters). Maybe it can be considered safe to include them under limited circumstances, but the Repugnant Conclusion indicates to me that the entities being similar to existing entities is NOT sufficient to make it safe to always include them.
(*) hedonic utility is extremely questionable imo - if you were the only entity in the universe and immortal, would it be evil not to wirehead?
Sure. But it will be my intuition, and not some impersonal law. This means it's okay for me to want things like "there should be some puppies, but not too many," which makes perfect sense as a preference about the universe, but practically no sense in terms of population ethics.