Kaoru AoiWhen to use each:
Use effect triggers when you can and event triggers when effect triggers don't apply.
Event triggers allow cards to watch for things happening to other cards. Effect triggers are predetermined when the trigger is set, and they typically predetermine common use cases. For example the discard effect trigger runs the discard effect on friendly cards in play when cards are discarded.
But more rarely you might want to activate an effect when an ENEMY card is discarded. Rather than make lots of discard effect triggers that are rarely used, it's easier to just use trigger on event to listen to the discard event, and this you can do from any card, including enemy cards, regardless of whether it was originally intended or not.
Trigger on event generally introduces more overhead and is a target for optimization if your game becomes too slow. That's because trigger on event activates whenever a matching event occurs, checking to see if the target matches the target the event it is firing on. If you find that it is getting troublesome, replace the trigger on event with a more targted effect trigger.