Ultimately, though, an HC gray is going to be the best choice for most folks with lighter rooms, and especially if the lights are on the sides. Consider the especially the Firehawk G3, and the various Screen Innovations Black Diamond screens (different gains, etc.) which are especially good, but also relatively pricey. Also: Elite's HC Gray, Da-lite's HC-Da-Mat, and so on. Typically we're talking screens with gains of 0.8 to 1.1 gain. The Epson's inherently pretty bright on all but the largest screens, so trading a little brightness for some ambient light rejection is a plus. (Note HC screens are a touch darker in the corners/sides.)
Don't get me wrong, you can go with a standard white surface, but in a light surfaced room, you'll also appreciate the gray surface's ability to lower the overall black levels.
OK, what about 3D?
3D Screens: With active shutter glasses based 3D, almost any screen will work. I'm told there are some slight differences, but haven't heard of any real issues.
With 3D, the issue is brightness - therefore, make your decision, based on the above, for either room, consider:
With 1660 measured lumens in our "tuned" Dynamic mode, the picture's pretty good in 3D! Figure that the image to your eyes is about 1/4 as bright in 3D. That makes our 1660 in 3D, the rough equivalent of watching 2D at 415 lumens. Which is to say, about 2/3 the brightness of watching 2D in best mode.
Thus, the Epson's 3D brightest mode has the juice to handle most of those 110" and 110" screens with good brightness, and be a bit dim with those 125" and larger screens.
So, in figuring your screen size. you might, if you want to max out for 3D, 9 (depending on your type of room - cave or not), opt for, say, a 1.1 gain, instead of a .8 gain.
Or, you might want a 1.3 - 1.5 gain screen instead of a 1.0 - 1.1 gain. The Epson's 3D image is as bright as any 3D capable HT projector we've seen, including the Panasonic PT-AE7000, the less expensive Epson HC3010, the Acer...
I'm not a fan of the brighter high power screens like the Da-lite 2.5 gain, too narrow a viewing cone for my taste, but if you want really bright 3D, some of you might opt for the best 2D screen choice and get a pretty high gain screen - manual or motorized, to use only for 3D.
Hope that helps.