The main similarities between them are that they are two brave, intelligent and adventurous characters, who strive to achieve their goals.
About the differences, we can start talking about their ages Jim is just a boy, inexperienced and who doesn't know much about the world. Silver is an adult man who has been through many things and carries many experiences with him. In addition, Jim is described as a kind, charitable and honest character, Silver, on the other hand, is a cheating pirate who spares no effort to get what he wants.