There is no difference between a commonwealth and a state in the U.S. To Locke, Hobbes, and other 17th-century writers the term "commonwealth" meant an organized political community -- what we today call a "state." Officially Pennsylvania, Kentucky, Virginia, and Massachusetts are all commonwealths. When Pennsylvania, Kentucky, Virginia, and Massachusetts became part of the United States, they merely took the old form of state in their title.
Today, commonwealth also means a political unit having local autonomy but voluntarily united with the U.S. These are Puerto Rico and the Northern Mariana Islands.
Pennsylvania was the #2 state to join the union.
Massachusetts was #6.
Virginia was #10.
Kentucky was #15.