It depends . Can you elaborate on what you're trying to achieve?
Errai Bus can be used for client local events (as an alternative to GWT's event bus) but it also bridges the gap between the client and server. In my opinion, there's nothing wrong with using both GWT's Event Bus and Errai Bus in your app.
I would consider Errai Navigation a simpler alternative to Place/PlaceHistroyMapper etc. but that depends on your requirements and design.
In fact, i would like to limit the use of many components to do the same thing so if Errai Bus is an alternative to the GWT's event bus, that's fine for me
About Errai Navigation, is there a way to implement page transition animation for mobile web app like m-gwt with Places ?