I use the 5DMkII myself, and have been pondering about the same question to myself.... This is my 2c:
Someone in this thread earlier on said that movement is better captured in a lower resolution camera; an argument which basically holds no ground at all. You have to take into consideration all the aspects of the sensor. One cannot compare the full frame sensor to that of the 7D. You will never get the same amount of detail that the 5D is able to capture, with a 7D. This I know for fact. A friend of mine, who had a 7D and recently bought the 5DMkII, shot a photo in exactly the same conditions as I had shot with my 5D moments before. We used the same lens, the same shutter speed, same aperture, lighting, etc. When the photos were compared at 100% zoom, the difference in detail and dynamic range was visible straight away.
Nowadays, it's not about pixels. It's not about having 30MP anymore. It's about how much detail you can get on that sensor of yours. I have been through the features of the 1Dx, and I definitely like what I see. You don't have to be a sports photographer to need the high frame rate; as mentioned earlier on in the thread. High fps is equally important during that dance, or during a speech.
ISO. OK the 5DMkII is great at pictures up to say 2000 ISO, even 3200...but I would never sell a print to my client which was shot at 6400, unless if it was a monochrome image with intentional grain. Being able to shoot natural light, handheld, in dark situations is a definite plus.
So my final word is this: The 5DMkIII will never match the specs of the 1Dx. It will def not match the ISO, fps, or focus performance. It will still be a pro camera, but it will NOT be Canon's flagship camera.
I think I will stick to my trusted 5DMkII for now, and wait for the 1Dx to hit the shelves. I will then buy the 5DMkIII in the future, when my 5DMkII has gone through its paces.
Oh, and by the way, one other very very important thing...irrelevant of which camera you go for, is the glass. I only use fast, L series glass on mine. I would never go for third party brands. It makes THE difference. If you ask me what decision I am most grateful I took in the last couple of years, I would not answer that it was the decision to buy the MkII... but my decision to buy the 70-200mm IS II USM.
...and the nifty fifty, of course.
So, my recommendation for you is that before you go for the 1Dx or 5DMkIII....first you should line up some high spec glass.