The 1D X is a bit better at high ISOs, slightly less noise (perhaps 0.5 stops at reasonable high ISO; it's more than that at ISO 51200 and higher, but I don't consider those usable anyway) and a bit more DR (2/3 of a stop or so). The 12 fps is certainly nice in fast-moving situations. The customization is a lot better, and the build is much more robust. If you use a grip on the 5DIII, the 1D X is better on a tripod. It's also heavier and bulkier.
I love my 1D X, and have no regrets that I didn't get the 5DIII instead. Is it worth it? Only you can decide that, depending on your budget and the opportunity cost of the difference in price.