When Plato speaks of aristocracy as a superior form of government to democracy, he is not thinking of the kind of hereditary aristocracy that was established in places like Great Britain and other parts of Europe, in which incompetent or immoral people often wielded power because they were born to it. Instead, Plato's aristocracy is the rule by philosopher-kings, who attain their position because of their wisdom, virtue, and personal merits. Unlike a traditional aristocracy, in which the aristocrat's power is based on the property he owns, Plato's philosopher-kings would own no property. That way, they could not be corrupted by greed and would rule in the best interests of all the people.
Plato prefers the aristocracy of the philosopher-king to democracy because he believes democracy allows people to do whatever they want, whether it is in their best interests or not. Democracy will, for example, lead to a choice of poor leaders, because the common people lack the wisdom and education to make wise choices about who can best guide them, preferring those who pander to short-term gains rather than electing those would attend to long-term prosperity. Democracy, Plato believes, would quickly turn into anarchy, with everyone doing what seemed best for themselves and often pursuing silly desires without regard for the common good.
As for whether or not you agree with Plato's criticisms, you will have to decide that for yourself, but you can use some of the points outlined in the above paragraphs to analyze Plato's viewpoint and compare it to your own. Good luck!