In addition to the details listed above, the members of the American Colonization Society believed in property rights, and that slaves were, in fact, property. In this vein, the ACS pursued a strategy of "compensated emancipation", meaning that they believed slaveowners should be paid for their property, and then the colonization of Africa with purchased slaves could begin. To them, it was a perfect solution: slaveowners got paid, and slaves were freed. It was also shortsighted and naive, but that's another thing.
The American Anti-Slavery Society, on the other hand, was not much interested in compromise, nor resettlement in Africa. It argued for complete, uncompensated and immediate abolition of slavery. Led by the firebrand, hardcore abolitionist and publisher of The Liberator, William Lloyd Garrison, the AAS held ACS members in some contempt, and had little tolerance for half measures like colonization.
Both of these societies are generally seen as anti-slavery, but only the American Anti-Slavery Society was a true abolitionist movement. By contrast, there were even slaveowners among the membership of the American Colonization Society.
The American Colonization Society was dedicated to the idea of sending blacks back to Africa. Some of their members were abolitionists who wanted the slaves freed and sent back. But others were slaveowners who just wanted free blacks sent "back" so as not to "contaminate" their slaves with ideas of freedom.
By contrast, the American Anti-Slavery Society was a full-on abolitionist group. It wanted slaves to be emancipated immediately.
So the ACS was not really about ending slavery (though some of its members were) while the AAS was completely dedicated to that goal.