I would go with a bag end, flat to 8 hz...
Well, maybe not, if you are looking at just those 2, go with the 15, chances are its more efficient and it has a larger frequency range. personally I would get a Klipsch or something, mostly to match my other speakers, but eh. Oh, and Klipsch doesn't really have any horn loaded subs, the have horn loaded lows in their heritage line and horn loaded hi, mid-hi, mid-low, and low drivers in their cinema line, but the subs are just ported. To get an effective horn loaded sub for the reasons you want a sub, it would have to be huge, even a large folded horn sub from a PA is really best till something like 40, otherwise they get too big to move around