If you decide to use a window, consider putting it on the south side, for the winter sun. it will allow sun to shine in and warm up the inside of the coop during the cold winter months.
I was just getting ready to recommend a window and then saw this post. One of the things I wish I had done when I built my coop was put in more windows. You need the light when you go to clean out the coop, gather eggs, work with your birds. And you can pop the window open - providing you have hardware cloth or some other protective wire to keep predators out - to increase ventilation.
And a window on the southern side is perfect for the reason stated above.