This is a good question, because what the book underlines from the beginning is the fact that they are different. One is big, the other is small. One is quick, the other slow. When it comes to similarities, two points stand out.
First, both Lennie and George are committed to each other. This is probably the most important similarity. They are the only ones in the book that show true friendship. This is why one of the refrains in the book is that they have each other. Here is a quote:
Lennie broke in. “But not us! An’ why? Because . . . . because I got you to look after me, and you got me to look after you, and that’s why.” He laughed delightedly. “Go on now, George!”
Second, they both also have the courage to dream. Surprisingly, there are no dreams in the book. In a world of brokenness, people do not dare to dream. Crooks says it best when he states that he has seen dreams come to nothing. This is not so with Lennie and George. They have a dream to get land and live off the fat of it. In fact, this dream drives them in many ways.
"O.K. Someday—we’re gonna get the jack together and we’re gonna have a little house and a couple of acres an’ a cow and some pigs and— "An’ live off the fatta the lan’,” Lennie shouted. “An’ have rabbits.