First: In my testing room, when I fill the 106" Carada Brilliant White with the MovieMate 85HD, it is a whole step up in brightness, compared to filling my larger (and darker) 128" high contrast gray screen in the other room. The combination of the smaller screen and 1.4 claimed gain is impressively bright compared to viewing in the other room. It's a far bigger difference, than toggling lamp power between high and low. So, if you've got the lighting control, then one decision is whether to trade more image punch for blacker blacks.
If any of you are using a masking screen that covers the letterbox area when you are viewing movies, you won't see the otherwise very dark grays that can distract, in the letter box area. With a projector like the 85HD, however, the smaller and higher gain the screen, the lighter they will be, of course, as the image will be brighter.