Julia is a pragmatist who hates the Party because it interferes with her ability to enjoy life. As Winston notes, "she only questioned the teachings of the Party when they in some way touched upon her own life." Winston, on the other hand, hates the Party for more broadly principled reasons. He fears the lies that the party tells and loathes the way its constant surveillance and thought control warps the people around him. He fears the dumbed-down world the Party is creating, no better reflected than in the character of Parsons. Winston hates and rebels on principle against the Party's censorship of thought. This more "ethical" stand, doesn't, however, make him a better character than Julia, just a different one.
The differences between Julia and Winston come out in their interest in history. Winston, whose job it is to rewrite news stories, is intensely concerned that real history not be forgotten. For example, he remembers a time when the Party did not claim to have invented the airplane, though now it does make that claim. He wonders how long it will be before the Party takes credit for inventing the steam engine. Julia, on the other hand, doesn't care about this at all: "the fact struck her as totally uninteresting. After all, what did it matter who had invented aeroplanes?"
In some ways, however, Julia's pragmatism makes her more perceptive than Winston. She is, quite cynically, prepared to believe, for example, that the war with Eurasia is a fraud: "The rocket bombs which fell daily on London were probably fired by the Government of Oceania itself, ‘just to keep people frightened,'" she tells Winston. He is surprised: "This was an idea that had literally never occurred to him." He also "envies" her for laughing at the two-minute hate.
The book suggests that both Julia's practical, day-to-day approach to fighting totalitarianism and Winston's bigger picture, more philosophically reflective concerns are important. To fight a cynical, corrupt system, you need to be a bit of a cynic yourself: Julia knows, for example, how to get supplies, like real coffee, and how to fake her loyalty, which are important ground level means to survive and thrive. Winston, on the other hand, can construct a good argument beyond the personal for why it is vital to oppose the regime. In the end, however, his ethics won't save him: he betrays Julia as readily as she does him.