Ralph tries to attain more of a democracy with his power, by instituting voting. He also seeks to maintain order by having the boys raise their hands when they want to speak and by having everyone "pitch in" with the work on the island.
Jack is an anarchist. When he gains power, he abuses it and sets up a totalitarian-like society. Under his rule, the boys are afraid of violent, physical punishment for no reason other than Jack's amusement.
Similarly, it is interesting to note that while Jack is much more enveloped by the dark side of human nature, Ralph certainly has his moments. From smaller issues, like teasing Piggy, to larger ones, like taking part in the murder of Simon, Ralph shows he is also capable of evil acts. Whether this is a result of power for the two boys is questionable, however. Golding tends to focus more on inherent evil rather than evil as a result of power.