My understanding of the difference between these two goes as follows...and idea of God doesn't really pertain here:
Mahayana- newer, more innovative of the two. The ideal or Bodhisattva here is that once the individual has achieved enlightenment, they turn away from Nirvana to aid in enlightening others. We are all one so we all must be freed from suffering for any one to be freed.
Theravada- leans towards simple, austere, and traditional. Arhats or saints follow the eightfold path (right views, right aspirations, right speech, right behavior, right livilihood, right effort, right thought, and right contemplation) in seclusion, usually in a monastary. Individual strives for enlightenment, Nirvana.