On what grounds are you comparing the two subs. I ask only because to evaluate the two, they should be judged in similar setups.
The same setup would be ideal. Also to make an unbiased evaluation, you will need to place the subs in the same location, level match them, correct any room induced frequency bumps or dips. Then listen to source material on both.
Logically thinking, the 15" should outperform the 12" if all else is the same. But like already mentioned in the thread, how the sub is constructed plays a major role along with quality of the amplifier used to drive it.
