ionic | Factory vs Service

Aqui ficam alguns sítios que explicam a diferença entre usar Factory ou Service no ionic e quando devemos usar um ou outro.

AngularJS : When to use service instead of factory:

http://stackoverflow.com/questions/18939709/angularjs-when-to-use-service-instead-of-factory

Angular service or factory?

http://iffycan.blogspot.com.ar/2013/05/angular-service-or-factory.html

The old Factory vs Service question
https://forum.ionicframework.com/t/the-old-factory-vs-service-question/17678