In new testing api when should I use lookup vs register? (mocking/stubbing)

[rondale_sc]

In the new testing api when is it appropriate to do:

this.owner.lookup('service:foo').someProperty = '123';

vs

import Service from '@ember/service';
this.owner.register('service:foo', Service.extend({someProperty: '123'});

I asked this in Ember.js community slack and wanted to document the responses from @TBieniek and @rwjblue

[rondale_sc]