Nothing DC or DD offers will get as low as the RE XXX (new model), but after that point there is not enough info to have a debate.
My 07 XXX15 was a low note monster, but wasn't very loud. It also required a huge SEALED box to even get as low as it was intended. I love the 05 XXX's, and I've had 7 of them total. I would pick those over the new one any day, unless your building for a home theater setup.
DD actually makes subs that sound good, and have a few SQ trophys if I'm not mistaken. DC, I can't comment on, but I'm sure they can hold their own too.
Install is 80-90% of the battle, so The actual sub used should be the last thing to worry about, and more focus should be on the actual application the sub will be in.
If all you want is an spl setup, then the XXX is not what you want. DD(9500) and DC(lvl 4xl and lvl5), will both do great in spl systems as well as competent daily setups. The choice comes down to what you want more in terms of purpose. A great sounding sub, that gets loud, or a straight spl/meter beast