There is no simple answer to that question. It mostly depends on games you play. You would need to determine, if your games are limited by current GPU or CPU, and swap the part that limits you in most games. Note however that you are most likely not going to see any performance boost in some of the games - those that were limited by the other component.
Unless you play at resolution higher then 1080p - in that case, swapping GPU first is obvious choice.