Go with the dedicated room, because:
- It will be dedicated (if you have domestic partner/roommate you can listen when and to what you want)
- It is almost exactly the proportions that Cardas recommends (and what I built for my dedicated room)
- Set it up as you like (speaker across the narrow wall and near-field listening is what Cardas recommends)
Don't know your speaker choices but have no complaints about my room (8 foot ceiling height is standard, at least here in the U.S.). Having built my room, I insulated the walls, ceiling, door, ductwork, and electrical (as much as reasonably possible). I run floor-standers that are rated 25 - 20,000 Hz, but am not a head banger. Near-field listening helps to eliminate room effects, is what studio setups use, and provides amazing imaging (sort of a blend between simple/dull far-field and headphones). But to work near-field requires the speakers to have an extremely coherent presentation (driver integration) to avoid hearing the sound jump from midrange to tweeter. With my particular speakers, room, and near-field setup I have treatment (6 GIK 244 panels) at front wall, front corner, and side wall 1st reflection points but they provide little improvement. James' setup should allow extra soundstage width, but also provide "extra" room-induced bass reinforcement, and could muddy imaging with those very short differences in direct versus back wall reflected sound waves.
My room priorities are: proper proportions; bigger is better; and design (decorate/finish) in whatever style you're (and your ears are) familiar with. Cubic/spherical shapes are absolutely horrible. Tiny rooms add room effects, tend to be cubic, and limit bass/soundstage. If you've (properly) shopped speakers for the intended room and it is decent there should be little need for room treatments. In my 8 foot x 13 foot x 21 foot drywalled room, beyond the GIK panels I've got 3 tall bookcases (2 on one side wall and 2 on the opposite wall) to act as diffusors (and of course hold books/supplies) commercial wall to wall carpet on concrete slab (basement) and that's it.
The best speaker design accounts for standing bass waves, inherent to every room (basic law of physics) and would include dispersed bass sources (multiple subs) to nullify the peaks/dips in bass response, so the main speakers would only cover down to mid-bass (80 - 120 Hz). Ideally you would have shopped for room first, speakers second to fit the room, then the rest of the gear to serve the speakers and your needs (space, formats, control).
