A journalist's loyalties should be to the public. A journalist writes for them, and there is an unspoken agreement that journalists will inform the public of what they "need" to know. A lot of times this need to know is really more of a want, but that's for another post.
Another aspect of this loyalty is that journalists should print the truth. In order for the public to be able to trust what a journalist says, all journalists need to do their best to uncover the truth of a story and print it accurately.
Public relations practitioners owe loyalty to their clients. A person in PR needs to look at the needs of a client and fight to help fill those needs. Although the loyalty is for a different group, truth is still important. If publics can't trust a PR practitioner, than that person is worthless to clients. However, a PR practitioner doesn't have the responsibility to inform the public of all of a client's dealings. There is a balance that each practitioner needs to determine on a case by case basis.
Loyalty is an important value to me both personnally and professionally. Loyalty can often determine a person's actions. When I make decisions, I often consider the people involved and how the decision will affect each person. I usually determine what I will do by deciding which person or group of people I owe my loyalty to.