Mark,Both the direct and the reflected window light will wash the image on either front or rear projection screens. Front projection screens are effected a little bit more because of "Gain", but any light getting to the back of a rear projection screen will also wash the image and whatever light comes by the sides of the screen from the rear will add to ambient light on the viewing side of both types of screens.
In your case I think the front projection would be slightly better. An increase in projection power (lumens) and contrast ratio of the projector will over come some of the loss, and a shorter distance to the screen will increase brightness but also make the image size smaller.
Rear projection grey screen would help with black levels and color brightness but that requires an increase in lumens also.
It appears that an increase in $$$ and a decrease in ambient light is your answer regardless of screen type.
Paul
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