It's an amazing game,and never before had a game left me sad. I am kinda uncomfortable about the ending. I have harvest a few little sisters just because some of them were located in tight places and trigger big dady to kill me. But I have save some of them. Kind a feel bad....
In the end the best option is to save all of them, after saving so many they'll leave you a present so the extra you may get the minute you harvest a Little Sister is quickly out weighed after you rescue a few and get their present.
Play Bioshock Infinite. That game had a mind blowing ending. Also if you a PS3/PS4 play The Last of Us. It's the only game that almost made me cry.