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Rescue or Harvest?
I've reached the point in Bioshock where it seems I have a choice to either kill or otherwise do something else (save?) my first little sister. I usually pick these moments in games to save, try one option, and then reload to try the next. However the game has cleverly denied me the option to save at this time. (Bastards!)
From some light browsing among other questions of similar nature I see that apparently saving all the little sisters gives you a different ending. My question(s) are thus;
Does saving or killing all of the little sisters effect my gained ADAM in any way?
Does saving or killing all of the little sisters make the game harder in any way? (other than the game being more difficult as a result of less aquired ADAM?
Is there any reason to save some little sisters and kill others?
Note that I literally know NOTHING about this game other than what information has been gleamed from my sole playthrough up until this point so please, keep answers as spoiler free as possible.