Do you kill francis or derrick on gta 4?

As you get no real indication in the game of which brother is the best one to whack, you really have to consider which character you think probably deserves it. If you kill Derrick you will be rewarded by Francis and he will help you with wanted levels once. If you kill Francis, you get nothing extra.

It also changes the Undertaker mission a bit, depending on which one of them you shoot.