These are the most difficult decisions to face in your life, but also probably the most important. Perhaps the situation they relate to isn't all that important, but the choices you make set a precedent for future choices. If you make a choice now that hangs on your conscience, it will make it much easier to do the next time. If, however, you make a choice to keep your conscience clear, it can also make these choices easier. The difference here, I suppose, is how you wish to lead your life. It may not seem like it with the individual choices you make, but they do add up over time. The accumulation of these choices is what ends up being known as a person, you. Each individual choice helps to mold that person.
I'm facing one such dilemma currently. On one hand, I can clear my conscience and provide a bit of a warning and openness to a friend... but at the same time, risk trouble for myself at work. On the other hand, I can keep my mouth shut, have that blemish on my conscience, and risk losing the trust of my friend. As cold as it may sound, friends come and go. But at the same time, so do jobs. The question is which one I would rather risk, and which has less chance of failing. The more important question, however, is whether I want that blemish on my conscience.