For one, Tybalt is Juliet's cousin, and he hates Romeo and all the Montagues. Mercutio is Romeo's best friend, though not a Montague.
Mercutio is related to Prince Escalus, which means he must be from a fairly powerful family, even more so than the Capulets and Montagues.
Tybalt is particularly violent, saying, in the first scene, that he hates even the word "peace"; Mercutio is essentially peace-loving, though he will employ violence in order to protect his own or his loved one's honor.
Mercutio is fairly intelligent, philosophizing about the nature of dreams and so forth, while Tybalt is really all about honor and strength and beating others.
Mercutio also likes to make really juvenile jokes, they're pretty crude and lewd, but he does have a sense of humor. Tybalt, not so much -- he really doesn't seem to have a sense of humor at all.
Finally, Mercutio has friends. He clearly hangs out with people whom he likes and who like him; Tybalt seems to inspire loyalty, but only, perhaps, out of fear. He doesn't have any "friends," per se.