A U.S. citizenship is one step above a U.S. permanent resident. It is one step beyond a Green Card holder. A Citizenship gives you the maximum rights available in the United States. US citizens may also find it advantageous to use a U.S. passport when traveling abroad. 
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Advantages of US Citizenship
Citizens have much greater ability to sponsor relatives for U.S. immigration. Many government contracts and jobs require a U.S. citizenship. You can not obtain a voting card without becoming a citizen. Click here to request a US Citizenship Information Kit.

• A citizen does not generally have to reside in the U.S. By contrast, Green Card holders can have their Green Cards revoked if they fail to reside in the U.S. If you become naturalized you do not have to be concerned about replacing your Green Card with newer versions. For example, the INS announced the expiration of the old Green Card forms I-551. All people in possession of the card had to apply for replacement with a secure, machine-readable Alien Registration Receipt Card. Citizens do not have to do this.
• Only American citizens can obtain a U.S. passport.
• Entering the United States is easier with a U.S citizenship.
• Many countries waive visa requirements for U.S. passport holders.
• With a U.S. passport, you are eligible for U.S. citizen services from U.S. embassies and consulates when traveling throughout the world.
• U.S. citizens do not have to carry proof of citizenship. On the other hand, INS feels that permanent residents must always carry their Green Cards. INS has detained permanent residents who forgot to carry their Green Cards.
• Ability to vote. 
• Immigrate other family members to the United States
• Prevent risk of deportation. A non-citizen can be deported for a convicted crime while a citizen cannot. 
• Eligibility for state and federal jobs.
• Ability to live outside of the U.S. and never lose citizenship.
• The right to run for public office.
• No paperwork of renewing their certificates of naturalization. 

Finally, there are intangible benefits. Most Green Card holders have decided that the United States will be the permanent home for their families. They find it of psychological benefit to be on equal footing with their American peers. This is important for e.g. school age children who are very much concerned with their identity.

For more information on how to apply for US citizenship, you may order an U.S. Citizenship Information and Application Kit  and start the application process right away. In addition to the U.S. Citizenship information Application Kit, you may request the U.S. Citizenship Test Study Guide.