There are quite a few key differences between the barber from "Just Lather, That's All" and Daru from "The Guest." Three of the most glaring are as follows:
The first and most immediate difference between the two is their level of interest in the conflict that surrounds them. The barber makes it known that he is secretly a rebel, though he avoids any explicit action that would indicate him to the enemy. He does this so that he can continue informing to the rebels what he hears around town. Daru, on the other hand, wants nothing to do with the conflict, and very clearly resents being forced to take part in it in any way.
A second difference is the nature of their temptation. Both are tempted to commit an act of rebellion, but the nature of these two acts is quite different. The barber is tempted to open Captain Torres's throat as he is shaving it. It is made clear that physically speaking, this would be an incredibly easy thing. Daru, on the other hand, is tempted to shirk his duty and set the Arab prisoner free. This is a different sort of rebellion, one against the nature of conflict altogether rather than one against a specific side.
The final difference is the consequence of their inaction and action, respectively. The barber, deciding that it would be cowardly and morally dangerous to kill Captain Torres in such a way, elects to shave him without incident. It is finally revealed, however, that the captain, on some vague level, knew that the barber might have contemplated killing him. Whether he is informed on by the captured rebels or even what Torres plans to do to the barber are not made known. However, it is understood that if the barber had acted differently, the consequences for the rebels and himself may have been different.
Daru's fate, on the other hand, seems almost deterministic. After mustering all his courage and his frustration with a conflict that would consume even the life of a schoolteacher, Daru defiantly releases the prisoner and tells him that his fate is in his own hands. It is implied, however, that the prisoner turns himself in anyway, and that his comrades will seek revenge on Daru all the same for turning him in. This brutal sense of meaninglessness is a thematic center for "the Guest" and indeed much of Camus's work. The barber's story is largely about the consequences of inaction, whereas Daru's is about the futility of action.