If you harvest the Little Sisters, you get 160 Adam. If you save them, you get 80 Adam, and then a 200 Adam bonus every 3 you save (along with a couple other gifts). From a pure Adam resource standpoint, you're losing 13.333 Adam per Little Sister. This amounts to nothing over the course of the game. And, again spoilers here, the game has a Good and Bad ending. If you harm the Sisters, even if you don't harm the majority of them, you get the Bad ending and have to replay the entire game to see the Good one. Meanwhile you can probably spare them all, saving before the last one, get the Good ending, revert to your save, harvest the last Sister, and get the bad ending.
It annoys me a little bit, since it takes effectively takes the weight out of your choices and, I feel, doesn't reflect an appropriate level of sacrifice to be "good".
