What is one's relationship with the world upon attaining Nirvana (and how do we work to get there)

There seems to be this ongoing debate both within the Buddhist Tradition itself and with the world-at-large as to how a practitioner should relate to the world. The Buddha himself forsook his family and all worldly-pleasures in pursuit of a spiritual life prior to his enlightenment. Such practice and attitudes has largely been preserved within the Theravada tradition. The Mahayana, in particular its Chinese strain, stressed on the ability to heal the world within and without, to relate to all sentient-beings in a loving way through the practice of Metta and Karuna - the Bodhisattva way of life. Compared to the Christian Gospel of love, the Buddhist doctrine seems so much more rational and pragmatic - Metta is just a means to an end, not an end in itself; it is practiced just to help the yogi remove obstacles from his or her road to nirvana. It is not uncommon to find Buddhists who are aloof to worldly affairs and loving relationships - the yogi who chose to lead a life of solitude and inner-peace, in place of worldly pursuits; the vagabond who chose to wander in this world so that he may find his true home in another. My question would be: How should an aspiring Buddhist aim to lead his life and relate to the world?