Yes, I think there is a difference between being kidnapped from your home, shipped across the ocean and sold into a life of servitude (with any children you have being born into the same condition, or yourself being born into such a situation) vs. doing labor as part of a sentence for a crime of which you have been duly convicted (and will someday be released from). That is my opinion.
Would your opinion change if the legal system that permitted people to be kidnapped, shipped, and sold, was the same system that decided if you're a criminal fit to be kidnapped, shipped, and sold?
No. The system that allowed the former was changed. I reject the premise that convicted criminals were or are "kidnapped, shipped, and sold" in any way that is comparable to chattel slavery. Were there some abuses? Probably. We live in an imperfect world.
