I have used both the window covers and GG polycro. Can't say for certain, but the material looks the same, weights approx the same thing when the same size, and seems to be similar in durability. Love them. Durability is good. My first sheet had around 60 nights before it was noticeably worn, but still serviceable. It hasn't been retired yet, though I normally use a slightly wider one so it's not gotten a lot more use. This is compared to me destroying emergency blankets in a couple of nights, and less than a week destroying the lightest painters drop I found / tried (I think they were 1mm, though they might have been 2mm).
Tyvek is nice. I think more durable that polycro and more breathable, so there is less chance that it will be soaked on the bottom in the morning. On the other hand, Tyvek is heavier, much more bulky, and not as waterproof. for me it's polycro all the way, but I understand why some people really like Tyvek.