I can speak to the performance but I can comment on when to use Components vs helpers.
Components are helpful in several scenarios:
Multiple elements that work together
When I was writing that code, I got to a point where I needed to write code that would check if certain elements existed on the page. By using the component, I was guaranteed that the elements existed and it was easy to handle their events inside of the component.
When wrapping 3rd party plugins
Here is a good example of this: http://stackoverflow.com/questions/18992613/trying-to-use-foundations-orbit-slider-in-rails-ember-app
Helpers are more like functions inside of your template. They’re great for modifying presentation of values, for example if you wanted to add $ sign to currency fields. This kinds of detail is relevant in presentation but not in the logic, so it makes more sense to create a helper rather than keeping that create another property in the view or a controller.
These are the once that I can think of.