The difference is mainly in the motivation. Bodhicitta is often translated as "compassion", but it's much more than that. It's about reaching Enlightenment for the sake of the others. Therefore, the methods also differ. Theravada practitioners will focus on reaching Nirvana, Emptiness, and once reached, will remain in it for eons! Mahayana practitioners will focus on helping others, wishing to reach omniscience for the sake of the others, reaching Emptiness as a means, not a goal.