15" vs 18" subwoofer - which to buy?


If price and room space/aesthetics were not a big issue, is there any reason to get a 15" subwoofer over an 18" one? My main issue is not disturbing the neighbors too much. I'm confused about the difference between a 15" and 18" subwoofer. I want to get the best sound quality possible for low-to-medium volume use.

I've heard that an 18" subwoofer can be played at low decibels and provide good bass resolution and fullness of sound, while to achieve the same volume with a 15" subwoofer, the power has to be much higher. So for any given sound volume, the main difference is in distortion- and higher distortion sound may be more obtrusive. Is this true? it seems counterintuitive that an 18" subwoofer could be better for neighbors than a 15" subwoofer.

However, I compared a B&W800 (12") vs B&W850 (15"). I listened from the other side of the store wall (not as thick as my apt), and at minimum levels which I found satisfying for HT, I found the 12" more obvious and 'boomy' sounding while the 18" produced a more subtle (though actually more powerful), lower frequency vibration, like a very low-level rumbling background earthquake. The 15" subwoofer did seem more of a disturbance because of it's 'obvious' sound. However, I worry that an 18" won't even start producing quality sound until a certain volume that was much higher than a 15", and consequently potentially more disruptive for neighbors. Is this true? What is the relationship between subwoofer size, low sound volume, and subjective listening experience?

Unfortunately, it will be difficult to get an opportunity to hear both the DD-15 and DD-18 which I am considering, so I hope someone can help out here.
no_slouch
I'm going to recommend something entirely different. Get 2 lesser subs rather than one big one. You can place them in such a way to cancel out certain room modes and get a much more even response throughout the listening room. I also think those with parametric eq built in is very useful, you can fine tune the one mode (floor to ceiling) that you usually can't do much about based on positioning. The Harmon Group wrote a great paper on number of subwoofers. There's a link to it on our resources page. There's also a paper we wrote on speaker placement, which includes subwoofer placement.
If it is a question of 2 smaller subs vs one larger one, I absolutely agree with Rives. The recommendation for large cone area assumes that cost is not the deciding factor (which I admit may be unrealistic unless you are building the subs yourself). Velodynes don't come cheap.

The loss of low frequency at low volume is mostly the result of human hearing characteristics, rather than actual reduction of SPL from the speaker system. Corrective action for this is the "Loudness" control that used to be a common feature of preamps.
Get the DD-15. I have mine set at 4 and have no plans to crank it up any higher.
My dilemma at this point is not about 1 vs. 2 subs. Should I get two larger 18" subwoofers then? Would larger subs would hinder room placement in my case? The difference is about four hundred dollars from my dealer between the two sizes, and I can afford two 18" models - cost is not an issue in the case.

Flemke, I have no plans to crank my subwoofer(s) either, I will play at low volume at least during the next year in this apartment. My question is whether the DD-18 can be played at an equally low volume as the DD-15, and achieve better effect musically. Unfortunately, I won't have a chance to demo both in the showroom.
http://www.velodyne.com/velodyne/products/specs/DigitalDrive.html
Look at the specs.