Disclaimer: I own neither of the mangas/animes nor their characters.
Warning: Spoilers as to how far you are into the manga and anime.
Sebastian versus Gilbert
Gilbert: Terrified of cats
Sebastian: Loves cats
Gilbert: Weapon of choice is a gun
Sebastian: Weapon of choice is anything he can grab
Gilbert: Has the power of his Raven chain
Sebastian: Has demon
Gilbert: Would risk his own life for his master
Sebastian: Takes his master's life
Gilbert: Part chain, part human
Sebastian: All demon, no human
Gilbert: Servant, yet heir to a dukedom
Sebastian: Just a servant
Gilbert: Over one hundred years old
Sebastian: Quite possibly over 1,000 years old
Gilbert: Amazing, curly, black hair
Sebastian: Amazing, straight, black hair
Gilbert: Slightly merciful
Sebastian: Completely merciless
Gilbert: Serves master due to loyalty
Sebastian: Serves master to eat his soul
Gilbert: Family oriented
Sebastian: Family? What family?
Gilbert: Lives almost forever
Sebastian: "Lives" forever
Gilbert: Sweet and cruel
Sebastian: Evil and cruel
Gilbert: Kills more than necessary without orders
Sebastian: Kills more than necessary with orders
Gilbert: Mentally unstable
Sebastian: Mostly mentally stable
A/N: This is an edit because the original one was very outdated. I will change again if needed.