Little Sisters were expanded a great deal in this game to feature a gathering mode where you will have to protect them against attackers. You can do this twice and follow it up with a save or harvest for the maximum benefits / ADAM. Otherwise you can head straight for a vent and save them or harvest them whenever they're on the ground.
I'm not certain what path gives the best ADAM gain overall because there are some genes which increase ADAM gain and one time ADAM bonuses in the care packages which Little Sisters gift you. If you harvested enough then Eleanor will give you 100 ADAM near the end of the game. I'd recommend rescuing because those presents come in handy and it gives you the good ending.
