I think you will be dissapointed if you really think the 1dx is going to be better IQ-wise.
Don't really think noise at 12800 is that much better than 5d3 to be honest. And a first quick test with some pulling of that iso 250 file doesn't prove very promising in terms of dynamic range.
This test done by a Dutch website clearly demonstrates that, in certain circumstances, the superior DR on the 800 can actually be used and seen: http://www.digifotopro.nl/content/canon-5d-mark-iii-vs-nikon-d800-dynamisch-bereik
In short, much more room for correction without killing the image.
In particular in cases where you want to push deep shadows and pull highlights, and do heavy dodge and burn in lightroom for example, I'm positive the D800/D4 chips will give you a clear benefit over the 5d3/1dx.
In a lot of situations you won't see sensor advantages however.