the 15 is pushing out more air and from personal experience jumpin from a 750watt 12inch W7 to a 1000 watt 15 MTX Sledgehammer9500 off the same jl 1000/1 amp man my 15 is hittin like i never heard. 15's knock man
Some times, but not always. Its more about the box, the box is what matters. That is how good quality it it, how well its sealed, how much volume it has, and what type it is. A 15 can pound hard, but its a lot slower than a 12, and a 10 is much more faster than a 12. Your choice of sub is dependant on pretty much the music you listen to. ie: Rap- deep bass- sounds best with 12's. Rock: fast drum beat, not any 'rolling' bass- sounds best with 10's or a couple of 8's. You get the idea.