When should I be using an Event Handler versus an Event Aggregator?
In my code, I have two ViewModels that controlled by a parent ViewModel, I am trying to decide if I should just use an event handler to talk between them? Or use an Event Aggregator? It is going to just be simple method call, I don't require parameters to be passed between them.
The way I see it, the EventAggregator is usually the heavy gun used when you want to publish an event to the entire application and more specifically - when you don't know who exactly is listening.
In your scenario that's not really the case, you have 2 view models that want to communicate, but they both know each other. So there is no real reason you can't use events.
I'll just mention that if you want to keep it a little more loosely-coupled - make an interface for each of the viewmodels that exposes the event. This way each VM will use the other VM's Interface instead of a specific instance.