Your only recourse is to appeal the Article 15. You were provided this opportunity during the hearing. If you elected not to appeal you can go back and ask to appeal but there is no requirement for the command to honor your request. In some cases the command may honor your request.
Punishments do not have to be the same
Keep in mind a commander is allowed to consider numerous factors when determining punishment under the proceedings of an Article 15. Some of these factors include: age, experience, maturity, duty performance, etc. There is no requirement that individuals who commit the same offense be punished in the same manner.