The first one (constructor) is related to the class instantiation and has nothing to do with Angular2. I mean a constructor can be used on any class. You can put in it some initialization processing for the newly created instance.
The second one corresponds to a lifecycle hook of Angular2 components:
ngOnChanges is called when an input or output binding value changesngOnInit is called after the first ngOnChanges
So you should use ngOnInit if initialization processing of your function relies on bindings of the component (for example component parameters defined with @Input), otherwise the constructor would be enough...
