To fully answer the question of Banquo's loyalty, you need to examine Act 2 also. In Act 1, sc. 3, after the witches make their predictions to Macbeth, he asks the witches for predictions for his future (ll.65-68). After Ross gives Macbeth the news that he's been named the Thane of Cawdor, he cautions Macbeth to remember that sometimes the devil, or evil spirits, tempts people with small truths in order to lure them into darkness. After Ross leaves, Macbeth tells Banquo, at the end of scene 3, that he'd like to talk with him sometime about what just happened (ll. 179-182)and Banquo says , "Very gladly." All of the exchange I've just summarized indicates that Banquo sees the witches as an evil entity and believing that, he probably realizes that what they'd told Macbeth is something that could lure him into trouble. In this case, the trouble is disloyalty. This leads to the notion that Banquo is more loyal to the king than to Macbeth because he seems to be cautious in dealing with the witches because he doesn't want to be disloyal. The first scene of Act 2 backs up that idea when Macbeth encounters Banquo out walking with Fleance late at night outside Macbeth's castle. Macbeth says he'd like to speak with Banquo about the witches prophecies and indicates that what he has to say could be good and advantageous for Banquo, "If you shall cleave to my consent, when 'tis, / It shall make honor for you." Banquo's response is one of caution. He says:
"So I lose none / In seeking to augment it but still keep / My bosom franchised and allegiance clear, / I shall be counseled."
This means that he'll listen to what Macbeth has to say as long as it doesn't require him to be disloyal [to the king]. Therefore, Banquo was more loyal to Duncan than to Macbeth.