Your screen size is not too large, my calculations based on your 25 square foot screen is that you will get around 32 ft-lamberts (800 lumens/25 square ft). The HC Da-Mat is probably the better choice for you. Even with it's 0.8 gain you will still get quite good brightness off the screen (~25 ft-lamberts).
I wouldn't recommend a high gain screen, you have quite a bit of brightness already and high gain screens inevitably suffer from some hotspotting (whether it is enough to be noticed is another question). Unless you are going to be projecting with significant ambient light, the HC Da-Mat will serve you better than one of their high gain screens. Neither the Da-Mat or HC Da-Mat will perform as well as a gain screen with direct ambient light.