More important is the relationship between you, as a Big Daddy, and the Little Sisters who still populate Rapture. As before, you can save the girls or you can harvest them to become more powerful. Not a difficult decision (well, depending on your moral bearing), but neither is it contrived. The greatest achievement of BioShock 2 is the import it gives to your choices. Protecting the Little Sisters is hard, much harder than simply killing them, and the immediate payoff isn't as great as it was before.
Instead, the importance of choosing the right path reveals itself over the course of the game. Defeating Lamb is easy. Passing along the right values to your Little Sister is hard. She's watching you, and learning. What kind of world do you want for her?