Neither is going to do a great job in a 6000 cubic foot room. Contrary to Deftech's claims, I think the Hsu will go deeper, although I think the supercube might be able to get louder at higher frequencies. Deftech's specs are vague, and probably a bit misleading. I don't think it would be a bad sub, however. Personally, I would go with the Hsu, just because I am offended by Deftech's dumb specs.
Like I said though, neither is going to fill that room with great bass. I would second Bandphan's suggestion of the ED kit, for the most bang for your buck. If you don't want to do it yourself, I would recommend the Epik Sentinel, it will probably have the most output for $700. The Hsu VTF3 or the Outlaw Audio LFM-1 EX would also be good. If you can up your budget by 100, I would suggest the Elemental Designs A5-350. You need as much output as you can get for that kind of space, with the Sentinel and the Supercube, you will sacrifice some extension to get there, but those subs will do more to pressurize the entire room, I am guessing. The Outlaw, Hsu, and ED subs should be able to get deeper bass though.