You really should talk with an attorney who has experience with both immigration and family law. Guardianship would not confer citizenship on your ward, while adoption likely would. However, the overlap of immigration law with adoption law means you should get specific, competent legal advice before deciding how to proceed.
Best of luck to you.
This response is intended to be a general statement of law, should not be relied upon as legal advice and does not create an attorney/client relationship.
I assume that you are interested in the child obtaining permanent residency status and U.S. citizenship through the adoption.*
From an immigration standpoint, your answer will depend on a number of things.** First where is the child for, how and when did she enter and what are the status of her birth parents. If she was from a country that has implemented the Hague Convention on Intercountry Adoption (HAC), you may need to complete the adoption in conformity with the U.S. law implementing the HAC. This might mean that the child has to return to her home country and be adopted under her home country's laws. Otherwise, the child may not be able to become a permanent residsent or citizen through the adoption, even if the adoption is completed in conformity with the law of the state where you reside. As a result, these areas need to be evaluated carefully before any adoption process is started.
International adoption is becoming more and more complicated. I strong suggest that you contact an attorney with experience in this area.
*Legal guardianship will not provide the child with immigration benefits.
**You will need to contact a local attorney in your state to discuss your state's adoption process and requirements.