There are very few times when a grey screen is appropriate.
Is your room painted dark, or is it white? Your window that you aren't properly covering is an issue as well. I'm not sure how the Silver Ticket grey screens match up against their white screens. Grey screens are a huge issue for some companies as it introduces artifacts into the image such as sparkling.
If you have a lousy room, then compounding a window, with light/white walls, white ceilings, and light carpet is going to a serious hindrance to the movie viewing experience no matter what.
Get a grey screen.
But, if your walls/ceiling/carpet is darker, then get a white screen. As long as their is no critical viewing with the window partly uncovered, you should be fine.
Understand though... A window that is uncovered will suck. It's going to cast light on the screen and you will see it. If you want light on in the room, then directional light, no pointed at the screen is the way to go. You can have a TON of light on in a room as long as it isn't directed at the screen and can't easily bounce onto the screen.
SEE:
http://www.avintegrated.com/lighting.html