Harvesting every little sister will net you more adam (160 each time, instead of 80 each time + 200 every 3rd.), where as saving every little sister (There is not that much of a benefit from mixing them) gives you some extra rewards like ammo, materials and some useful plasmids/tonics like hypnotize big daddy which is rather useful on higher difficulties.
I recommend saving every little sister to get the good end and
simultaneously unless you're going for multiple playthroughs (which is not needed for 100% if you do it efficiently)