Don't have the X-13 and only one X-12, which is enough for my house.

Looking at the math you could get two X-12's for less than one X-13 and the two would have almost 30% more radiating surface area though less total amplifier power than the X-13 .

So if the extra couple of Hz extension and the THX certification don't do much for you then I'd be tempted to go for the two X-12s. The one thing that may make it worth it is that the X-13 should have less distortion.

Oh and the X-12 is about 40 lbs lighter

That's my 2 cents. As always YMMV.