Well, keeping in mind the best interests of the child, the factors which are coming to my mind right now are the child’s age and wishes, what will be the parents wishes , also what will be the ability of each parent to take care for the child, the mental and physical health of each parent and the child, what is the interaction with siblings or other persons taking into account the child’s cultural background, also if there is any history of child or spousal abuse, the child’s adjustment to the current home and community, and the length of time the child has lived in a constant environment etc etc...
the list is quite long.. these are the only ones which i could think of at this moment.

Need some more ?, check the law books, my dear !