Hi Meridius,
How big are you projecting? If the screen size is too big, then you will need a screen with gain. The more gain, the higher the black level, so you have to decide if the image is too dark overall and if the black level can go any higher. Lamp projectors will never 'project' true black anyway.
A screen with gain will have a viewing cone - the greater the gain, the tighter the cone, but if you're not moving around it may not be noticable. Those at the side will get a dimmer image than those in the center (up. down, left or right) but depending on the gain and where they're sitting, they may not be getting a great deal less than a matt white screen. If you're off to the side by quite a bit, then they could be seeing less than they would on a matt white screen or a white wall. It's something that has to be taken into account really.
When you see screen gain on a manufacturers literature, you may see the trem 'half gain'. This is where the screen falls off to half the gain of the advertised gain, and is given in degrees:
The 1.35 gain Stewart Firehawk gives 0.68 gain at 24 degrees for their 1.35 gain grey screen:
http://www.stewartfilm.com/engineeri.../firehawk.html
If viewing angle needs to be taken into account for your situation, then try to find this data to evaluate the screen.
Gary,