There really is no such thing as a "textbook" sealed subwoofer. All subwoofer designs involve tradeoffs, whether it is size/cost/features/etc.
Comparing a relatively compact sealed box subwoofer to a larger and more expensive ported subwoofer that has 2-3x more internal volume and takes up much more floorspace is really an apples to oranges comparison. Even so, the ULS-15 can more than hold it's own with respect to deep bass extension, frequency response linearity, and feature set.
Things start to get more interesting when you add a second ULS-15 into the mix. With TWO ULS-15, the max clean output capability is as good or better at most frequencies compared to most very large ported subwoofers, with some very tangible benefits that multiple subwoofers can bring to the system. Two ULS-15 have at least 8-9db higher max clean output capability (using CEA 2010 standard) at and above 25Hz compared to a VTF-3 MK3 in extended bass mode, while having deeper bass extension and a much more shallow low frequency rolloff too. In addition to that, multiple subwoofers can help one smooth out in-room frequency response. And since each ULS subwoofer is relatively compact, has wireless connectivity, and weighs less than 90 pounds, they are much easier to move around and place in-room compared to a large ported subwoofer.
Kain, based on your room size/dimensions, two good subwoofers should give very tangible benefits in terms of frequency response linearity and low distortion vs a single subwoofer, provided that you have a flexible enough budget to accomodate the pricier dual sealed box system. If not, just go with one large ported subwoofer (or two lower cost ported subwoofers, which may be as good or better than a single larger ported subwoofer, depending on the design and the application).