I've owned both subs and don't own either now. I am going to buy another one of them though and I can tell you i can pretty much buy any of these subs, price makes no difference to me. I will be getting a Magnum 15" (again)
The magnum is WAY louder with 2000w. Magnum has WAY more bass above 40hz. Magnum is cheaper (for those who do care about cost), Magnum is better for music like Trance, house, etc. where the xxx just doesn't have the upper bass impact to do it.
You won't get a xxx to pound you teeth out with 1500w either. I had 2000w or so on mine and it was loud but not that loud.
I had the magnum 15" tuned to around 35hz for a while with 35 square inches of port and OMFG was that loud and clear!!!!
I have to say the xxx is probably more accurate but an incredibly accurate sub is not what most people think it is goin to sound like. Its a mellow sound, with no super rock hard upper bass impact like the magnum (which is probably more realisitc to real life). I find that everyone that is into bass that got into my car (not big time SQ guys) 100% prefered the magnum over the xxx besides bass below 35-40hz because it is a monster below 40hz thats for sure. Magnum is at least 2-3 times more efficient.
With 2kw people were turning their heads to me at every stop like like WTF WAS THAT!!!!!!!!!!!!!! with the magnum ported, with the xxx it got loud but not enough to turn heads. But I did prefer it for total raw SQ, but its in no way a "fun" sub IMO, cept for deep lows.
as you can see i am not biased in any way. I can buy just about any sub I want no problem and the Magnum is probably my favorite. H2 was pretty inefficient for my taste, and needed eqed ALOT ported to get it to sound really good. Sealed it was great. But you need alot of power to get it SUPER loud, and with that kind of power it'll blow so you need 2 IMO.