Well, I'm not sure I agree with everyone. I recently demo'd (quite extensively!) 1.6s compared to 3.6s, and the fact is that the ribbon is everything people say it is (suffice it to say I have the 3.6s). And audio is all about trade-offs, so here is my take:
If you put them along the 18 foot wide wall, you will be closer to them (some say this is how it should be anyway, but if you get them 4 feet from the wall and assuming you don't sit 'in' the wall, then your nose will be about (13 ft - 2 - 4 =) 7 feet from the speakers, and if you get away with 3 feet from the front wall, then 8 feet. I sat 7.5 feet from mine tonight and it creates a sort of Innersound effect - less wall reflections, more direct sound. A bit less fullness and/or width of soundstange, but incredible clarity - possibly unbeatable even up to 20k... You can't turn them up at all, they are almost like headphones like this :) - but in terms of clarity and transparency, it is breathtaking. Note that Mapleshade actually recommends more of this equilateral triangle position versus the more traditional isosceles for those exact reasons. So its about trade-offs. If you have them 3 or 4 feet from the wall they will take up a lot of your room, is that okay? Would family or roomates be okay with that?
If you are putting them along the other wall then I think we have more problems that are less surmountable. They are 2 feet wide. So lets say you want them 2 feet from the side walls, that leaves (13 - 4 - 4 =) 5 feet inbetween them. And 2 feet certainly isn't optimal, many have said they need to be a bit farther from side walls for bad reflections. At 3 feet it gets funny (13 - 4 - 6 =) 3 feet inbetween. That certainly would be odd to say the least.
So, depending on the wall, I think a case could be made for the 3.6s, given certain tradeoffs.
Also, you will move at some point, and these speakers are 20 year or longer type of investments... I'm currently in an 18 X 19 room, and feel that this is a bit tight (but the front half is double high, which helps) and one speaker is in front of a stairway which certainly isn't optimal, but we will move at some point. And I have the ribbons to listen to in the meantime. You can see from my Audiogon virtual system how one speaker is in front of stairs, and the other in front of a short hallway. They still sound quite amazing despite this 'unoptimal' positioning.
If you look at bookshelf speakers (although I'd second the MMG recommendation above before bookshelf speakers) my short list includes: Tetra 120U, Von Schweikert (don't know model number, their site just changed and they may not sell them anymore, but if you can get them, they are great), SoundLabs MiniStat (have never heard these, but would suggest checking them out), or Mark and Daniel Ruby or Topaz - all of these are very 'particular' and interesting speakers.