It would depend on their age and how old they were when you and her married. Think about making an appointment with an attorney to discuss what option would be best. If they qualify, it would be faster for you to file as a US Citizen. Good Luck.
Is there any reason you felt compelled to put US Citizen in quotation marks? If the children are under 21 and were under 18 when the marriage occurred, your application would be possible and efficient.
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The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.
if the relationship with your spouse occurred while the children under the age of 18 they are considered under the immigration law as your stepchildren. As long as they are under 21 when you file the petition they may come to United States as immediate relatives. Your wife may also petition for the children directly however the fastest way for the children to come to United States is through a petition by the United States citizen stepparent.
The short answer would be that you should petition for your stepchildren because they are your immediate relatives, ESPECIALLY if they are overstays. This can be a complicated matter, so you should consult with an immigration attorney, or you may make a situation worse and more expensive.
If the children qualify, it would be quicker for you to file for your step-children, however, you should have an immigration attorney review your case to determine if your step-children qualify and if they are eligible to file for their green cards.