We are told a great deal less about Tybalt than we are told about Mercutio because we see Mercutio in many more scenes than Tybalt. Since we know so little about Tybalt, it is difficult to find very many similarities, but one thing we do know is that both characters have quick, fiery tempers.
We do not see Mercutio's fiery temper come into play until his death scene; however, we are told by both himself and Benvolio that he is very quick to quarrel. We also know that the name Mercutio is symbolic of the metal mercury, which is the most changeable metal in existence; it especially changes easily when heat is applied. Hence, we know just from the symbolism of his name that Mercutio's temperament changes easily when things get heated. In Act 3, Scene 1, Mercutio ironically describes his own temper in a string of witticisms accusing Benvolio of being quick to quarrel. We especially see the ironic accusation in the simile,"Thy head is as full of quarrels as an egg is full of meat," meaning that Benvolio's head is as full of quarrelsome thoughts as an egg is filled with yoke (III.i.22-23). Benvolio, whom we know is a peacemaker, confirms the irony in Mercutio's speech by saying:
An I were so apt to quarrel as thou art, any man should buy the fee simple of my life for an hour and a quarter. (31-33)
In other words, Bevolio is saying that if he really were as likely to quarrel as Mercutio is then any man should buy his inheritance. Hence, we see that even Benvolio agrees that Mercutio has a fiery temper and is likely to start a fight.
Likewise, we also see Tybalt as being as equally eager to start a fight. We not only see this in Mercutio's death scene, we especially see it in the first scene when he quickly jumps to the conclusion that Benvolio is picking a quarrel with the Capulet servants, when in actuality Benvolio is trying to stop the fight. We see that Tybalt has jumped to a false conclusion in the lines, "What, art thou drawn among these heartless hinds? / Turn thee, Benvolio! look upon thy death!" (I.i.61-62). Tybalt's ability to jump so quickly to a false conclusion concerning violence certainly shows us that Tybalt has a fiery temper.
One difference that we see between Mercutio and Tybalt is that Mercutio is shown to have a more sensitive side than Tybalt. While Mercutio does not approve of Romeo wallowing in the pain of rejected love, Mercutio does show that he is concerned about Romeo's state of mind, showing us that he is sensitive to his friends' feelings. We especially see Mercutio expressing concern for Romeo's state of mind when Mercutio expresses concern at not having seen Romeo since the ball and speculates that the reason for Romeo's absence must have to do with Rosaline, as we see when he says:
Why, that same pale hard-hearted wench,
that Rosaline, torments him so that he will sure run mad. (II.iv.3-5)
The phrase "hard-hearted wench" shows us that Mercutio feels that Rosaline is treating Romeo poorly. Also, the reference to Romeo's state of mind shows us that Mercutio is very concerned about Romeo.
We do not likewise see any of Tybalt's thoughts and feelings with relation to the other characters, so we cannot say that he is as equally sensitive as Mercutio. Hence, we see that one similarity that Mercutio and Tybalt share is their hot, fiery temper, while one difference is that Mercutio is sensitive while Tybalt is not.