Rescue or Harvest?

Solution 1:

All 53 tonics for the tonic collector achievement(if that's what you're after) can be collected in a different way other than purchasing it from a gatherer's garden. here is the complete list + where you can get each one:

http://faqs.ign.com/articles/817/817677p1.html

Oh and answering the question, rescuing them has always been better for me since you get more free stuff + Tenenbaum's help. But that's just me

Solution 2:

The way I understand it, either you get all the tonics by saving every little sister (because Tenenbaum has some tonics you can't get anyway else) or you get more adam to spend ON tonics (by being a cruel, child-hating adam-junkie).

So, because Tenenbaum offers unique tonics, you must save all of the little sisters if you're interested in collecting all the tonics. (For the other achievement, I presume?)