Regarding relative brutality, I recall a study of a Roman estate that showed that each year they bought half as many slaves as they already had, didn't sell slaves, and had a roughly constant number of slaves over many years. Do the math.

Wow. Was the estate training gladiators? (The only kind of roman estate I have any familiarity with and the only kind of industry in which that death rate wouldn't be a sign of abysmal management even neglecting all ethical considerations.)