A pair of XXX 12s and a 13w7 have about the same amount of linear displacement, but the 13w7 has far more linear suspension travel.
Given adequate power, they should be roughly equal.
The 13w7 is going to require more power than the XXX 12s to reach the same output level.
People foolishly assume that two 12s will outdisplace the 13w7, only because of ignorance of its capabilities.
The 13w7, with ADEQUATE power, will embarass most 12s. In this case, the two setups being compared are roughly equal, but you'll pay a lot more on average for a pair of XXX 12s than a 13w7.
The single ported 13w7 will require less space than two ported XXX 12s, and cost less.
If you have a large enough amplifier, the 13w7 can certainly be competitive. So basically, which do you want to give up? Space and money, or amplifier power?