Well that's a tough question. They are close enough that I go back and forth. I like one better some days and vice versa. The QSC seems more the truer FRFR solution, but with that big ole horn, I need to tame the highs a bit with EQ. Can be a bit harsh, but that is the nature of the beast I guess. The K12 is a little louder...so if you are going to only go with one, and headroom is a big factor, then the additional SPL advantage of the K12 might be a decision factor. To me, the FBT sounds more like guitar cab, probably has to do with the coaxial design. The highs are not as "ice picky" and are easier to tame. I have found that the bass and treble control are a nice option and I will tweak kind of like I do the global EQ depending on how I am sounding on any given day. The K12 seems to provide more thump and feels a bit more "spongy" to me for lack of a better word.
I love the connection options with the K12. My band plays Baba O'Riley and Who Are You by The Who and we use backing synth tracks for these, and I just run these through my ipod through the RCA input and there is plenty of volume to mix well with the LOUD band.
I like both of these options, and like I said before on any given day, I favor one over the other. But I have used each separately/alone live and have had equally pleasing experiences. There isn't one clear winner. They both can hold their own in a loud rock band context. 2 is nice, but diminishing returns.
However, I am not too happy about the problems I am having with the QSC unit though. Prior to having these, I owned a pair of Mackie SRM-450's and they are notorious for their thermal shut down issues which really gave them a bad rep. But I never had one problem with them. So, I just hope that my experience with the K12 is a fluke. I have not heard of any other folks having problems and there is that nice 3 year warranty.
So, clear as mud now right? With all of the EQ options of the Axe-FX and the cabs there is no reason you can't dial in incredible tone with either of these options. You can pretty much dial in/out anything that you want/don't want.
At the end of the day, I can't prefer one over the other, they each have their pros and cons. If one stood out significantly enough for me, I'd dump the other and go for a matching pair. You can always do like me and be an indecisive f*** and get one of each.
Scott