Mark, I'm not sure what side of your fabric would provides the highest gain -- the brand of blackout fabric I purchased also had two white sides, however, one side was smooth and "vinyl like" and the other side was more textured "fabric like"...anyways I tested both sides and ended up with the smoother side.
You might want to wait until your projector arrives before attaching the fabric to your frame. Test out both sides in different lighting conditions and from different seating postiions. Pick the side that gives you the best perceived picture.
Best of luck with your setup!