If you got a few tt's or CDJ-1000's and you have to choose out of only one of those two mixers and nothing else, than with the DJM-800 you would get most features for your money in your set up.
If you already have one or two Pioneer EFX-1000 effectors, or planning to buy those, you should go for the Xone-92. That is if you want to have more features, not for the mixer only, but for your whole set up. Because the Xone 92 has 2 filters and 2 send/returns, and the DJM-800 only one of both. However I think the controls of the filter of the DJM-800 are very nice. I mean the sound colour knobs for each individual line channel. The both Xone 92 return channels also have 4 band EQ and are fader controlled. And that's a plus with regard to performance capabilities. But the whole set up becomes very expensive ofcourse.
For production purposes you should look at what sound you're looking for: clean and clear digital sound easier to accomplish with the DJM-800 or the warmth of analogue, which you can accomplish with both.
Just remember that a mixer is not the only component in the whole sound chain. So there is always room with both mixers to achieve any sound. Maybe MIDI will be in favour of one of both mixers. But I don't know that much about MIDI.
Personally I like both sounds, analogue and digital, it all depends on the mood or music style of the track if you should go for clear and crisp or warmth or maybe even distorted. I don't want a world without one of both.
That's why it is an endless discussions. What somebody likes is personal. But it is true, that with the facts and figures, you can compare both mixers with regard to sound quality.
You just have to recognize the difference between sound quality
and personal sound taste
There is really a huge difference between those two words. Just read again if you still can't see the difference.
A set up with good sound quality you can adjust pretty easily to your own taste. But a system with bad sound quality you can never adjust to your taste.
And you can adjust the original sound of the producer if you don't like the original sound. There is no law against that. And it's very hard for me to believe that you can ever hear the real original sound of an artist, except when you're in the same studio where it was recorded.
For above reasons I think the comparison of both mixers is an endless debate as I said before. It all depends on the requirements one needs to do the job, which always varies in different situations.
The DJM-800 has almost everything imaginable. But still not everyhting that exists. So there really still is no perfect mixer that has it all.