The parties are different, not in how Gatsby has arranged them, but by what happens at the two parties, namely Daisy. Gatsby continuously throws such lavish parties because he feels that wealth can help him achieve his dream, Daisy. He wishes to relive the time that they had together so many years earlier. In Chapter 3, Nick learns of this. It isn't until Chapter 6 that Daisy and her husband appear at one of his parties. We see that she is unimpressed by everything as it is not the life that she is used to. All that Gatsby does for his parties (e.g. gathering the juice of 200 oranges each week) is futile. These parties are for his dream, for Daisy. In Chapter 6, we see that his dream is impossible.
The party in Chapter Three is simply a depiction of the Jazz Age, a tawdry picture of a time of hedonistic pleasure in which liquor flows, people unleash their desires, and wealth and plenty are recklessly displayed. In Chapter Six, the party has a more oppressive atmosphere as a contemptuous Tom is present, and even Daisy is "appalled" by the "raw vigor" of West Egg. In this chapter, Gatsby is not absent from the party, and he grows as an archetypal image of the Romantic hero.
The guests at the party Gatsby holds in Chapter Three are uninvited except for Nick; they whisper of Gatsby, a "testimony to the romantic speculation he inspired." Further, the guests speculate about Gatsby's true identity and rumors abound. People behave recklessly, such as Owl Eyes, who has been drunk for a week and whose car ends up in a ditch. But, in Chapter Six, Daisy "saw something awful in the very simplicity she failed to understand"--romantic possibilities totally absent--She stares at the people who are uninvited and Tom demands "Who is this Gatsby anyhow?...Some bootlegger?"
Chapter Three's party is clearly just a party; however, Chapter Six's party is a beginning to the recapturing of the past for Gatsby,
He looked around him wildly, as if the past were lurking here in the shadow of his house, just out of reach of his hand.
He talks with Nick later,telling him, "I'm going to fix everything just the way it was before...She'll see."