But the rubber meets the road in this aspect: Thou Shalt Not Kill. Abortion is the killing of a child who has God's gift of life in it.
There is no "Thou shalt not enslave." Nothing of the sort. Slavery is not killing, either. So for someone to try and compare the two makes no sense to me. At best, we're talking about a violation of freedom or civil rights with slavery, which cannot compare to the deprivation of life itself.
[And as an aside, I don't understand the slavery reparation people, either: if they want compensation for past crimes, then the way to go might better be related to the murders of relatives that took place, which have no statute of limitations, and in which the harm is indisputable.]
So I would not entertain the debate, for if handled on the terms of your opponent, you would have to accept that "apple=orange" from the very start.