I cant really comment on which is more durable as I dont have any long term experience with real shade cloth, But I can say I have fiberglass screens that are going on 10 years and they still haven't fallen apart.Tony, do you find the screen to be more durable than shadecloth? It seems like it might be. I guess I could ask, if you could get a roll or 40% shadecloth or a roll of screen that gives 40% shade for the same price, which would you prefer?
I do have a small, 10'x10' section of my original shade house built in august 2009, that I have 60% shade cloth on that I got from a friend with a nursery.
It was inexpensive and I needed something at the time I was building so I picked it up. It is ok for the youngest seedlings and epicactus, but I have to either sell them or move most seedlings out of that section in about a year if i want them to continue to grow the way I want. I would like to change it to 30% or 40% but have neither the money or the time to do it now. This shade cloth is woven so it doesnt unravel when you cut it and is still in great shape.
I would say if you are just covering a small space, go with the window screen for now and when your setting up your nursery and need to cover large shade houses go with the shade cloth.