The 3.6 is better suited if you are looking to go the Magnepan route.
6 responses Add your response
The short answer is yes, either speaker could work and sound quite good in a room that size. You might want to experiment with the ribbon tweeters being either on the outside or inside to get the best balance between focus/details and size depth/width of the soundstage. I have heard many Maggies sound terrific in similiar sized rooms. Hope this helps.
I have my 3.6's set up in a dedicated, treated room, 17 X 23 x 10. Speakers are oriented on the long wall, 4 feet out from the rear, about 8 feet from the listening position, 7 feet apart inside edge to inside edge, with the tweeters on the outside, with a slight toe in. There is about 3.5 feet behind the listening position as well.
If I do say so myself, I am getting wonderful results. Plenty of bass, and enough distance between the listener and the panels to give the sound room to gel.
The long wall orientation is a bit unorthadox, but it was based on measurements and models done by Rives Audio.
My initial impression is that while the 3.6 might work in your situation, provided you can experiment with equipment positions, the 20.1 may over-pressurize your room, given the requirement of the loudspeakers amplification needs.
The above statement comes with a qualification: small chamber works and small jazz ensemble works may be fine, but you might be pushing the envelope (literally), with large orchestral works and high-power rock with the 20.1.
The short answers are: maybe for the 3.6s and no for the 20.1. Perhaps not what you want to hear but here are the reasons.
The problem is with the short axis (12 1/2") of your room. You do not have enough room laterally to place your speakers along that short wall. So you must place them along the long wall as Slipknot1 does in a 17 x 23 room. With the speakers 2 feet from the front wall (a minimum really) and your listening spot 2 feet from the back wall (a must for good sound), you are now only 8 1/2 feet away from large panel speakers, which should be about 6 to 8 feet apart. 8 1/2 feet may or may not be enough distance for the sounds from large panels to blend properly--slipknot1 has 5 more feet than you do, which is critical for the sound to blend.
It's a very close call in your room for the 3.6s but the 20.1s are probably too big. The limiting factor is your short wall. (The same size room with 14 1/2 x 18 dimensions would have been ideal for these speakers). With lots of tube traps, you may be able to place the speakers closer to the walls and give them enough distance from you to make a great sound. Try before you buy.