A "normal" screen will have a gain of 1 (meaning it reflects light equivalent to a white painted wall; higher gain = more light reflected; lower gain = less light reflected).
Most ultra short throw (UST) screens with their amazing ambient light rejection (ALR) have a gain around 0.6 making them inherently "high contrast" screens which is a fancy way of saying they reflect less light and thus make blacks look blacker...the trade off is if you don't have enough lumens your whites will looks a touch dim. It seems the LS820 was designed to be paired with these lower gain UST screens and based on reviews looks a little over bright on a "normal" (gain 1) screen. Having said that, the LS820 can be set to dimmer settings and be peachy by most people's standards.
Alternately, with your light controlled setup you may want to get a simple (non ALR) "high contrast" screen which has a lower gain (around 0.6 to 0.8 is pretty common). It will give a similar color and brightness profile to the UST ALR screens but (obviously) lack the ALR features of the UST screens.
The BCP100 and 120 have a gain of 0.35... which is REALLY low. They have an ALR rating of over 90% however, obviously in pictures and tradeshow videos you can see of the LS820 in action with the BCP100 it works out peachy and has enough lumens to punch past that super low gain, so it gives you an idea how over bright it would be on standard settings on a normal screen with gain of 1. But it does leave me a little nervous that it will still have enough lumens at 120" on the BCP120. I'll be getting one in shortly and can comment directly about it at that point.
Long story short:
* with no ambient light, an ultra short throw (UST) screen vs. a "normal" "high contrast" screen with a lower gain (around 0.6) are going to look VERY similar, but you can probably get away with a higher gain and still get a great picture while using lower lumen settings on your LS820; presumably saving more laser life span. Also, non ambient light rejecting (ALR) screens are relatively inexpensive (just make sure that it is PERFECTLY flat, any ripples/waves/etc. will be VERY obvious when using a UST projector).
* with ambient light (with the lights on), an UST ALR screen will reject most of it allowing you to continue using the projector with the lights on. The BCP100 and 120 in particular (along with crazy expensive flavors from Screen Innovations and Pro Display, and the cheaper but size limited elite screens CLR) work exactly as was described to you, reflecting to you mostly the light coming up at the screen from the very sharp angle that the projector is using. Thus, sunlight, overhead lighting, etc. is severely negated. If you have floor lighting or a highly reflective or bright floor material near your projector; that light will still partially wash out your image. For most situations that is a trivial matter.
Important to know:
if you are getting the LS820 and BCP120 at the same time, work with view sonic directly to bundle them and get a discounted price vs. ordering online etc.
Even shorter version: if you don't care about using your projector with the lights on, get a normal screen with a gain around 0.8 that is perfectly flat(no wrinkles, ripples, etc.) use the lower lumen modes on your projector and call it a day. If you care about using your projector with the lights on... it's pretty hard to beat the BCP120 without breaking your wallet to smithereens.
Hope that helps.