A perfect benevolent dictatorship is nearly impossible to construct as there are always humans wanting ultimate power that will then use their existing power to break the poll-system of the dictatorship.
A perfect democracy is impossible, too. (as seen above a perfect poll system is just possible in political systems without any construct of power as anarchy but not in power-based systems as a representative democracy)
As there are more people to decide and people can make mistakes there are more people to vote wrong because of not having spotted a mistake. thus a benevolent dictatorship with a centralized mistake-spotting system is better for deciding right.