I just tested it out and it seems that if you harvest a little sister you get 160 adam out of her. If you use her to harvest 2 bodies you get 40-40 each plus you get 80 more when you rescue her at a vent. So it seems that it changed slightly from bioshock 1 where you got less if you rescued the little sisters. Plus if you keep rescuing them you get presents, so is there any point in harvesting them apart from the time you lose when you adopt them and walk around?
You do get more ADAM if you harvest any given sister. However, there are long term benefits to saving at least some of them.
The ADAM gained from the bodies is independent of whether you harvest or save the little sisters. You can always harvest them after they've sucked out the ADAM. So that way you get at least 240 ADAM per sister. On the other hand, like in the first game, you get presents for every 4th one you save. In particular, the 1st set of presents contains the Proud Parent tonic, which increases the ADAM obtained per body to 60, so with that, the amount per sister increases to 200 for saving and 280 for harvesting (after collecting from 2 bodies, of course).
The ending of the game also depends on how many sisters you save/harvest. And of course it's also portrayed as a moral choice. One of the achievements also requires saving every little sister along with other story related decisions.
For more details, see http://bioshock.wikia.com/wiki/Little_Sister#BioShock_2 .