Winston is so new to the whole rebellion thing that his attitudes towards it are much less developed than Julia's. Julia has been rebelling for years now, and has had a lot of time to really think about things, and form an opinion about them. She is much more educated on the strategies and purposes behind the propoganda than Winston is. Winston is just unhappy and angry, but hasn't really been able to define exactly how the Party has caused it. Julia knows how the Party caused it, and why they did it. For example, when they are discussing marriage, sex and relationships, Winston never really thought that the Party had funneled all of the passion and intensity that used to be put into relationships and love for their own purposes. Julia believes that the Party uses the intensity love and passion and stifles it between people so that it will be channeled into fierce and frenzied patriotism for the Party. People need an outlet for that passion and intensity; normally, our relationships or hobbies get it. In their world, the Party manipulates it into fervor for the state, or love of the Party itself. So, Julia definitely has more in-depth opinions and reasoning on the Party itself, whereas Winston is frustrated and wanting to do something, but not really understanding fully the implications of the Party itself.
Another difference is in how Julia and Winston want to rebel. Julia is an expert at covert, undercover rebellion. On the surface, she lives her perfectly patriotic life, but survives is able to remain happy through her relationships with men and finding ways to rebel within the system. Winston is happy with this for a while, but he wants more. It is Winston that truly pushes them towards seeking out O'Brien. It is Winston that has grand desires to join up with the supposed shadow rebellion that will overthrow the Party. It is Winston that gets frustrated witht he apathy and complacency of the thousands of Proles, that if they would just rise up, could overthrow the Party easily. He has grander ambitions that go beyond simple covert rebellion as a form of survival. Julia goes along with him, but it is Winston's desire that really comes through in that front.
I hope that helped; good luck!