- In Romeo and Juliet, Tybalt and Mercutio are both hotheads. They instigate the violence in Act III that leads to Romeo's revenge and exile.
- Both characters are passionate defenders. Mercutio defends the critics, those who rail against. Tybalt only defends his family.
- Both characters die in Act III, a turning point in the tragedy that sets up the tragic action for Romeo and Juliet in Act V.
- Mercutio is an archetypal Agroikoi, a sub-eiron, a Critic of Tragic action who resists tragic movement; Comic relief. As a foil for Romeo, he refuses both love and hate, adopting a cynical middle course. By contrast, Tybalt is all hate, melodramatic, overserious. He is an archetypal villain, a warrior with no ideals. He is the emblem of family allegiance.
- Mercutio's death is the primary foreshadower. Before his death, he curses both houses. As such, he serves as a kind of oracle. Tybalt's death is all action, no commentary.
To me, the main similarity between these two characters is that they both kind of like violence. Tybalt is always ready to fight. His first words and his last words in the play are about fighting. He has to be restrained from going after Romeo at the ball. Mercutio also is kind of hot headed. In the scene where he dies, he cannot wait to get at Tybalt and fight him.
The difference between them (at least so far as we can see in their words) is that Tybalt has no thoughts except violent ones. By contrast, Mercutio is funny -- as in how he makes fun of the Nurse in Act II, Scene 4. He also talks all kinds of silly stuff in his "Queen Mab" speech in Act I, Scene 4.