I think they're equally harmful for different reasons.
Self-loathing will have your view on the world colored by the lack of self-worth you have. And color those around you who interact with you.
Narcissism, same problem, opposite issue, since you won't have a good grip on reality because you love yourself so much.
Self-loathing affects group dynamics more, since people IMHO tend to be affected more by someone being depressed/down more than they are with them being high on themselves.
$0.02 where appropriate.