It depends on the application. In medical tests False Positive (FP) is preferred to False Negative (FN). Because if a person is labeled to be positive (sick) then additional tests can be taken to confirm if it is really sick or not. But if the person is labeled negative (not-sick) while this is false, then no action will be taken and this can be harmful. Alternatively, in other applications FN can be preferred to FP. For example in email spam classifier, FP will result in missing some important email but FN will be better because user won't miss such valuable information