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
Before you buy, check out the Revel Sub 30 system. You will get parametric equaliziers, a 1,000 watt amplifier, 15inch woofer with additional 15 inch dual-layer metal dome acoustic radiator.
For a review go to http://www.avrev.com/equip/revel_sub30/index.html
or Harman Specialty Group
If you want are interested contact Adidadi.
I have one Bag End Infra 18. It will shake the pictures on the wall. I recently purchased (2) conventional 12" powered subs from a local Phoenix dealer (Celestial) to augment it.
The Bag end goes low, I want to have more fleshed out bass above 20 Hz. The Celestial subs have the often used 12 NHT driver.
I also have a B&K REF 30, which is very underatted, in my opinion. This processor has very excellent bass management that allows you to notch filter & boost the bass, for your room. Anyone who owns this processor & doesn't appreciate the bass management, hasn't used it to it's full potential.
(I still have a dealer friend who tweaks mine with the on screen adjustments - whenever I make a room/system change).
I asked Tyler acoustics for a quote on a custom subwoofer with the following design parameters:

purpose is to use with single ended tube amps and very efficient speakers....the sub must be "quick" enough to keep up with demanding bass passages and the mid/hi frequencies.

dual 8" drivers arrayed vertically
one speaker input straight to the drivers
one speaker input with a quality passive crossover (cut at 60 to 75hz)
non ported design
high efficiency so can be driven by a tube amp if desired (intention is to build a mono tube amp that will only operate in the bass frequencies and opimtized for bass duty)..output control on the amp would control the level

Tyler quoted me a price of 450 for a basic black ash woofer based on these specs and estimated the bass output would go to 30hz or slightly below.

An AudioAero dual 8" sub which supposedly hits 20hz recently sold very cheaply locally. I missed it by a couple of days.
This is supposed to be an excellent product but that is only hearsay.
I think that for LF, and especially for SW, the more cone area the better, although, as Sean says, there are other imnportant considerations. The large cone area is not to play louder. It is to play with the proper loudness without need for extreme cone excursion. A small subwoofer must be very loud one foot in front of it because the sound pressure must spread out to fill the whole room, with the SPL falling off rapidly with distance. If people used multiple 15" subwoofers (one for each speaker system) there would be less chatter about the difficulty of locating SW in the room.
Eldartford you make a good point. I know some don't understand the concept of multiple subs in a system.
I think you hit the nail on the head. This is why I have went this route.Blind folded it will be nearly impossible to point out the subs in my system.