I would say that the ULS-15 mk2 will likely provide more SPL for the dollar than the L12 for starters. If you ever moved the sound system to a larger room, the ULS-15 will handle it just fine, whereas the L12 might struggle. Also, since you would not be running the ULS-15 as hard under any scenario, that means the sub suffers less stress, which in turn means the sub will last longer, everything else being equal. A dual ULS-15 mk2 system would likely be the last subwoofer system you will have because it is unlikely to leave you wanting for more, but a dual L12 system might leave you thinking about an upgrade down the line, which might mean selling it and taking a loss for something superior. I would be done with it and go for the ULS.