I think that when a photographer takes a Mirror image, they are focusing on emotions. The photo has a meaning. Usually, it is referring to the way the photographer feels about something. These images has a romantic feeling. On my opinion, I do believe that the photo tells a story when it is meant to be a Mirror image because I reflects emotions. The story can be about the person who took the photo and his or her emotions, and feelings towards something. on this photographs you can see passion.
If we talk about Window image, I think the photographer is focusing on the possibilities one might have or face in a close future. Meaning that the image do not necessarily has a story or emotion to tell. The image is there for you to see what is out there and its open to opinions. The image becomes a window of possibilities. In other words, these photos can be realistic and about real life events or settings.
There is also a philosophical difference. If a photo is a mirror in the sense Szarkowski means, it implies that the world can only be seen through someone’s senibility. In this same system, if a photo can be a window, it can be objective and report on the outside world.