When to use observers and one-shot systems?

[DasLixou]

I can't find any resource / short explanation about when to use one-shot systems and when to use observers.
One-shot systems seem to be ordered in the schedule so observers theoretically act faster, but what is the real "when to use what" thing?
Should I always use observers as long as there isn't any important ordering needed? What would be such an important ordering?
And what about the additional Bundle generic on Trigger? At the first glance it looks like a fancy special thing for component hooks, but wouldn't a generic OnAdd::<MyComp> also do it's job? And can this be used for other / custom events?