They were both inhumane and tragic. The holocaust happened in Europe where slavery (specifically black slavery) happened in the Americas. I think the holocaust resulted in more deaths though and if so that'd be my answer.
Both were horrific incidences that occurred in the history of this country. A deliberate genocide of over 6 million Jews and 50 million Blacks marred the heritage of both cultures. Good reason(s) to study history. Re-think and Learn.
Which was worse, hurricane Katrina or hurricane Sandy? One was primarily a black phenomenon, the other a white phenomenon. But both were expensive in costs of money and life, and taught us the limits of governmental resources. Both show us that an extreme disaster can have limited responses, and that it takes time to recover. But the holocaust resulted in the formation of Israel. Slavery did not create a nation. Ending slavery did give us the Civil War, but nazism has resulted in Israel - and the continuing middle east problem, with 4 wars and continued bombings as a proximate result. It's hard to say, but I think a case can be made that the Holocaust is a bigger issue than slavery. Slavery would have ended anyway, as a result of the invention of the Cotton Gin; it would have become economically unnecessary by 1880 anyway. The Holocaust was unnecessary, and the horrors of it pale any comparison with slavery.
Slavery lasted for centuries and still we find in some countries though illegally but it still exists. Holocaust came and gone. Yes any loss of human life and sufferings should be condemned by all means.
Honestly they both sucked, but i really think the holocaust was worse. Jews were rounded up to be exterminated. I didn't hear of slaves people being rounded up to be killed during slavery. Slaves were sold to white people and other black people. Jews were loaded into trains and shipped off to camps to be put into gas chambers, shot in the head, or to just be have crazy doctors experiment on them while they were still awake and able to feel pain...