The screen surface may offset a lower brightness/higher performance projector. I have never seen it in action, but the 1.4 gain Screen Innovations Black Diamond screens claim to perform amazingly in high light situations (even outdoors!!). Also, the smaller your screen (i.e. the closer your projector), the brighter your image will be. The 0.8 gain screen may work too, but I have read that it has some pretty poor viewing angles compared to the 1.4 gain one.
I would, if the Black Diamond screen performs as they claim, guess that your projector choice would not matter near as much if you had said screen, and you could probably look into the JVS RS10, or for less money and a bit more brightness the Panasonic PT-AE3000U (or soon AE4000U) or the Epson 6500UB (or soon 8500UB). Any of those options with the Black Diamond screen will likely cost (much) less than any upcoming 70+" flat panel display.
I would recommend investing in some good blinds as well, though it sounds like you don't want to darken the room - if the Black Diamond screen performs as advertised, you wouldn't have to....