First this question may be better answered in the diy section as I don't believe many on this forum have built their own subs and most haven't heard a horn sub.
The ultra is a very good subwoofer. There is no getting around that. There will be trade offs for going either way. First is cost. You already have the ultra so as it stands now its a free sub. It will dig lower than the THT and isn't as big either. You also have no effort to do anything with it since its already built.
With all that said, the THT will offer a few things the ultra can't do. The THT can get super loud. From what I've seen people routinely reach 120 db with that thing at near all frequencies it can produce. It is pretty cheap also. With unfinished wood you're looking at ~$400-500 depending on amp choices. Plans are $15 and if you have any problems you can talk directly to the designer Bill Fitzmaurice on their forum or here. He frequents this site. Your finishing on the wood has unlimited options. Only time and wallet restraints will hold you back from the most exotic finishes. Building it should be pretty simple if you have any woodworking skills. And the thing you pointed out, the matching with the la scalas, is a big deal. I have a pair of la scalas and also have a horn subwoofer. Not the THT, but still I have horns on every channel. My sub is a danley dts-10 that I built and it is insane. I'll give you the ultimate in a man's seal of approval...my wife hates it. 130 db in my 4000+ cubic foot room.