Can't comment from experience on the effectiveness of either one in a subwoofer application, but one of the manufacturers I represent uses Black Hole 5 in the "hot-rod" versions of his full-range speakers.

In a subwoofer enclosure, I'd be inclined more towards making the enclosure walls super-stiff, to drive the resonant modes above the subwoofer's passband, and then putting some polyfill in the box to eat up residual lower midrange energy. If the crossover is low enough and steep enough, you might not need any polyfill. But I don't think either Black Hole 5 or Sonic Barrier is going to do much at subwoofer frequencies.

Duke

Steve