If you compare 2 subs by the same manufacturer typically the 15 will be a larger, more powerful, more expensive version of the 12. It will have a larger amp and more cone area and will be "more powerful and deeper" than it's lesser counterpart. You need to compare specs of different brands. Some manufacturers 12's will handily outperform other manufacturers 15's. You can't just look at the size of the driver to determine if one sub is better than another. There's a lot more that goes into the equation.
Exactly!
So I've demoed some very high end brands, Paradigm, Velodyne, B&W, Sunfire, etc. None of the 12's have impressed me. My HT system is all Klipsch, it would be great to timber match with a Klipsch sub, but unfortunately at 15" sub is not available.