Not an historian on this subject, but it should be pointed out that there is no agreement that there were Jewish slaves in Egypt. We know for a fact that there were slaves in the Americas, and we know about their treatment.

So, I would say the demonstrably true situation of American slavery wins.

The thing with slavery is that every manifestation of it is awful. Treatment did vary but it's essential to understand that all slaves were subject to sale, punishment, or even death at the whim of their master. Aside from the Bible, we do not have any other sources that back up the existence of Israeli slaves in Egypt. On the other hand, African slavery has many sources that describe treatment. It really makes comparing them hard.

Why scholars argue that transatlantic African slavery was particularly bad is because of the racial aspect of the slavery. African slaves were still subject to reenslavement even when they were free. In addition their descendants suffered from the legacy of anti-black racism bred by transatlantic slavery. Hope this answers your question