Julia cares more about having fun and enjoying her life (her little trists with all her man-friends) than she cares about bringing down the Party. If fact, Julia thinks Winston's ideas about rebellion and his faith in the Proles is a silly waste of time.
Winston is obssessed with the past (the paperweight,the old songs, books, pens, inventions) and the beauty of the way things were before the Party took over and subjected everyone to their overly controlling rules and laws. The Proles, to Winston, are an excellent example of this. They are free enough to sing and enjoy life beyond the stifling control of the Party--it is with these people who can love, sing, move around and speak with whomever they please that Winston puts his faith. They are the ones who will rebel and overthrow the Party.
They both rebel in their own way, but they do not have the same ideas about how that should occur or what should happen once the Party no longer exists.
Julia and Winston do not understand the Party the same way even though both have ill feelings toward it and would like to see it destroyed. Winston is at least two generations older than Julia and he remembers snippets of what life was like before the induction of the Party so he can see more clearly the lies that are being spewed by the Party and it confuses and infuriates him. Julia is young, in her twenties, and knows nothing about life before the Party. She has been taught, and believes wholeheartedly, that the Party invented aviation. It angers Winston that she doesn't care that it is a lie. Julia wants to bring the Party down because of her rebellious nature and the fact that she doesn't care for the absolute power they possess, Winston wants to bring the Party down to expose the lies.