I'd go for a 12 if you like the deep stuff. Use the extra space available to make your box a ported one. And for the most part, a 12" will not tend to be "sluggish." You might start dealing with nonlinear (aka distortion) with an 18" burping at twice it's RMS, but for these applications, ever sub will be just as fast and have the potential to be as accurate as the other. The only advantage to 10's is that you can take advantage of coupling and gain a few dB's.
And I have seen 4 10's hit over 140 dB (TermLab) at 10 htz, so it really just depends on what you can fit/power!
