Is there a contrast between Trofimov and Lopakhin in Anton Chekhov's 'The Cherry Orchard"?

When you mention the "main character", I'm going to assume you are referring to Madame Ranyevskaya. (Checkov's plays tend to be ensemble pieces, and I'm not sure it's useful to designate a main character, but Madame R. may qualify as the central character in that everything revolves around her, and she is the symbol for the vestigial aristocracy that was at the center of the social upheaval taking place in Russia around and after Checkov's time.)

Regarding Trofimov vs. Lopakhin

• Trofimov is an idealist because he is young, inexperienced, and has a lot of free time
• Lopakhin is a pragmatist because he has more experience, and comes from a lower social class without the luxury of leisure

Don't forget that, towards the end of his life, Checkov declared his plays farces and they are undoubtedly comic, which accounts for their bittersweet quality. Most great tragedy contains comic elements, either wordplay or irony, but Checkov merges the two forms better than most.