Fahrenheit 451 and 1984 are both dystopian novels. What are their three main differences and three major similarities?

There are a number of similarities and differences between Fahrenheit 451 and 1984. Beginning with the similarities, both novels tell the story of a male protagonist who starts out as part of the system. Winston, for example, works for the Party in the Ministry of Truth, while Montag works for the government as a fireman. 
Secondly, the governments of Fahrenheit 451 and 1984 both use violence to maintain their authority. In Fahrenheit 451, this takes the form of the Mechanical Hound, who chases down its prey before delivering a lethal infection. Similarly, in 1984, the Party uses torture and execution to ensure that Party members operate within their extreme rules.
Thirdly, both governments do everything in their power to destroy freedom of expression. In 1984, for instance, Winston keeps a diary but knows that if the Party should ever find it, he will be sent to a forced labor camp (in the best-case scenario). In Fahrenheit 451, the banning of books has prevented freedom of expression, creating scores of people like Mildred who lack emotion and any deep connection with those around them.
However, there are also some key differences between these two novels. We see clearly, for example, that oppression comes from different sources. In 1984, it comes from the Party, a totalitarian group which rules with an iron fist, while in Fahrenheit 451, Beatty claims that censorship came from the people.
Secondly, oppression in Fahrenheit 451 comes from censorship, but people are generally free to live their own lives. They can walk around freely, they can associate with whomever they choose, and they are free to pursue their own interests. In 1984, however, the Party does not allow its members to have such freedoms. They are constantly watched, for instance, by hidden microphones and the telescreens, and they are forced to participate in mass activities, like the Two Minutes Hate.
This leads us to our final difference between the two novels. Montag, for instance, is ultimately successful in fleeing the Mechanical Hound and the city. As he watches from the railroad, he sees the city being bombed and realizes that there is a chance to rebuild society. In contrast, Winston is not so lucky. He is captured and imprisoned in the Ministry of Love, in which he undergoes a program of reintegration.
Montag, therefore, is successful in his efforts to bring down censorship, while Winston fails and learns (the hard way) to love Big Brother.