Tybalt and Benvolio are different because Benvolio always uses his sword to keep the peace and Tybalt uses his sword for battle and death. The differences between their personalities is quickly seen at the beginning of the play when the first street fight breaks out. Benvolio draws his sword and says, "Part, fools!/ Put up your swords; you know not what you do" (I.i.57-58). Tybalt then enters and seeing what Benvolio has just done and said, says, "What, art thou drawn among these heartless hinds?/ Turn thee, Benvolio; look upon thy death" (I.i.59-60). The conversation continues with Benvolio suggesting peace and Tybalt exclaiming that he hates all Montagues and any talk of peace.
Later, when Tybalt seeks out a duel with Romeo, Benvolio suggests that they all leave the public view of the citizens and go discuss the matter elsewhere. Tybalt refuses to listen and approaches Romeo when he comes. Then, at Romeo's request for peace, Tybalt refuses again to let Romeo go without a fight and says, "Boy, this shall not excuse the injuries/ That thou hast done me; therefore turn and draw" (III.i 62-63).
Clearly, the main differences between Benvolio and Tybalt are their desires for peace--or lack thereof.