Often the parents are able to determine this for themselves, either before getting to court or after they attend the Mandatory Family Court Services mediation. Children in their teens are often asked their preferences, and the courts will often go along with the child's preference. Each parent can make rebuttals to the comments made by the other parent. The mediator will assist the parents in reaching an agreement. Assuming that each parent has about equal parenting skills, then the mediator will look for the parent who is more child centered, and will likely base an opinion on the parent's attitudes, when recommending a custody agreement.
Mediation is absolutely critical.
The results of mediation are more often than not going to determine to outcome of the custody aspect of your case. It is therefore vital that you be as well prepared for mediation as possible. It may be appropriate for you to seek out professional assistance in preparing for mediation. Your attorney should be able to guide you in this area. I live out of state.