Nothing will beat the 1D X for fast-paced sports photography.
The 5DIII beats the 1D X for pretty much every other type of photography, especially weddings.
And for weddings, a pair of 5DIIIs will blow a single 1D X out of the water.
The 1D X has 12 FPS and (somewhat) better autofocus performance and AF-point-linked metering.
The 5DIII has a silent shutter, more megapickles, is smaller and lighter, and is at least 90% as good as the 1D X on all other specs, and it costs half as much as a 1D X.
If you're shooting for Sports Illustrated (or if you want to shoot for them), you'd be a fool to use anything other than a 1D X.
If you're shooting almost anything else, you'd be a fool to go with a 1D X over a 5DIII.
Oh -- and, if you're shooting for Sports Illustrated but you're shooting golf or tennis, you'd again be a fool to go with a 1D X over a 5DIII (because of shutter sound).