And projector is going to be at the back wall?
I guess, technically it is the "am" now, but I thought of something I had to get notes down on for myself, so I got up to do that and... well you know the rest... here I am.
Here's my advice... screen size sounds good to me. It's about in the middle of where I was and where I am.
I went with the 4k, because well, it shook out to be the best in a lot of people's opinion, and screen technology doesn't change that quickly, if at all, over time. So I decided to get the biggest, best technology, no holds barred, AT screen I could find and do it once and be done with it. No complaints at all.
The 4k material is specifically called "Enlightor 4K" by Seymour Screen Excellence, or EN4K for short. It is a joint venture that Seymour AV is involved in. The website is www.s-se.us
I had the 4k material up with my old AE4000U from Super Bowl until a week ago, and had no problem lighting it up. I just got the AE7000U a week or so ago, and it will almost burn your retinas in Eco mode, and my throw distance is ~18 ft on a larger screen, so similar to yours... don't worry about that. My Room length is ~22', less 2' for screen wall, less 1' for depth of projector... 18' throw.
I don't think there would be a difference for 3D between the two. The XD could have a problem as you mentioned with noticeable weave, or loss of perceived resolution at closer distances or the upcoming 4k projectors. Chris Seymour posted a while a link to an article about projecting 1080p source material by a 1080p projector and by a 4k projector and it was pretty interesting. So even if it takes a while to get a decent amount of 4k source material, you can still see a significant difference using 1080p material through a 4k projector. So I wanted to get as future proof as I could with the screen purchase I hope to keep for a good long time.
The SSE EN4K screen and frame are all fantastic. Heavy duty, not going to warp or bend, extruded aluminum wrapped in black velvet. Really a classy product.
One note... if you are thinking of doing masking for Scope images, mention that when you order everything. They are magnetic masking panels, and metal bars need to be built into the frame during production for the magnets to work.
Hope that helps.