It's often personal preference.
ISF techs argue that 1.3 to 1.5 gain is better for color accuracy, but unless your screen is very small, or you have a 9 inch CRT projector in a very dark room, you have an 80% or 90% chance of greatly preferring the extra brightness and contrast (despite the slight hotspotting) that a high quality non-sparkly 2.1 to 2.5 gain screen offers, like the Draper M2500. If you just have a 7" or 8" CRT projector and a room with white walls, then an M2500 screen will often stand out as being greatly preferable. Even if you have a dark room and a 9" CRT projector, you may still even prefer the M2500.
If you are going to sit under the projector, your viewing angle is probably 15 degrees or less - which is good enough for a Draper M2500.
The M2500 also allows someone to almost double screen surface area and still get the same brightness per square feet as a 1.0 to 1.3 gain screen.
If you're not sure what you prefer - try out a few screens out (expensive), get some screen samples from the manufacturers (Draper, Stewart).
David Bott, of the home theater pictured at www.avsforum.com
, tried out four different screens before settling on the Draper M2500.
[This message has been edited by Mark Rejhon (edited October 12, 1999).]