Well, the 1.3 gain will increase brightness, but at the expense of lighter blacks. Do you plan on watching a lot of 3D movies with the 7800D? The 1,500 lumens in 2D would work well with a 1.0 gain screen, but if you plan to watch a lot of 3D, I would go with the 1.3 gain as 3D viewing will decrease brightness.
My Home Theater
Screen: DNP Supernova BLADE (08-85)
Projector: Espon 8350
Receiver: Yamaha RX-671
Speakers: DefTech 8060-ST