Comparing China and Rome to a modern economy is basically impossible. Those were agrarian societies whose net output per unit of territory was largely static, and determined by the amount of food that can be grown in the land. I don't think it's valid to categorize them as either socialist or capitalist societies. Marx agreed: he wrote of feudalism (and probably, by extension other agrarian civilizations) as a distinct kind of society that predated the capitalism that rose out of the industrial revolution.