Best one of Plato's works to compare to Marx's Communist Manifesto and Sir Thomas More's Utopia?

Hello, I am writing a paper for school which entails a compare and contrast essay of fifteen pages in length. I am writing on the depiction of a Utopian society, and marginally dabbling into social justice/injustice. I have already selected Sir Thomas More's utopia as well as Karl Marx's Communist Manifesto, which one of Plato's' works corresponds best to these tow and would make a nice compliment to my essay. I was thinking either Crito or the Republic. I am only allotted with a time frame of three weeks, so anything obstreperously long is out of the question. Any answers appreciated, thanks.