This may not be very helpful as my room is smaller (12 x 18 with 5' knee walls running on the 18' sides) and I never got to hear the in-wall version, and you said floor-standing rather than stand-mounted (Klipsch doesn't have THX floorstanders afaik) BUT....
I can definitely vouch for the stand mount THX Ultra2 7.2 set. I honestly can't imagine much sounding noticeably better in a blind side by side in an appropriately sized room. Perfect tone and timbre matching all the way around is just so convincing.
That said, the in-walls are probably great too. I would expect it's a trade-off for the space savings in the room, but I haven't heard them to say for sure.