Rescue, because there is no logical reason to harvest.
At first I thought harvesting gave you more adam, but this is not the case. From what I remember, saving a Little Sister gives the same/more adam as harvesting them. The difference is that you have to wait for a present containing the adam to appear near a Gatherer's Garden, but that's a very minor point. It's not like getting the adam right away has any major advantage. But rescuing the little sisters also gives you access to the hypnotize plasmid, which is normally unobtainable. So harvesting has no benefits in comparison, meaning there is no reason to do it.
This is actually my biggest problem with Bioshock. The moral choice system is painfully shallow. The only two options are "Kill" or "rescue", and it is very hard to justify doing the former. Adam is not as vital as the game claims. I beat the game on the hardest difficulty, no vita chambers, mostly using the wrench. With just a few plasmid buffs this is all you will need for most of the challenges. You get more than enough plasmids just by searching the environment. So even if harvesting granted more adam, it wouldn't make the game much easier. But it doesn't anyways. You have to choose to be an evil bastard, since being good has no disadvantage. So your morals aren't challenged, which was the whole point of this system.
