The point isn't to find which is worse, but to slay the fallacy that slavery = american slave trade when in fact it is a permanent fixture of human society.
Discussion
The point is that American slavery had some big differences and heavily contributed to the strength of the American empire which ended up kind of running the world post World War Two. Which is why people talk about it so much, I don’t recall the Arab slavers establishing world dominating empire and carrying a lot of those methods and concepts forward, yet.
And saying it’s permanent makes it sound like it had monolithic characteristics and not affected by historical events, but don’t mind me.