there isn't much similarity in the murders other than both being out of power-hungry ambition.
for Duncan, Macbeth is confused about weather he really wants to kill the king, he doesn't feel as if he could live with the guilt and is worried about being found out. but with some urginng on from his wife he commits the murder. his conscience and emotions are still in play at this point.
for Banquo, he is a coldhearted killer now who doesn't think twice about murdering his close friend and makes all of the decisions about this murder on his own. L.Mac does not find out until after the killing is done. he is completly blinded by power and sees it as a necessity to remove this "obstacle"from his path to glory.