I'm very much in the market for one of them. The 5d3 would be a great replacement for my 5d2 but would the 1dx offer even more? Would it save me time in post by having more accurate exposure and white balance (?)? Would I keep it longer due to the extended shutter life? (probably not )
It's a complicated equation.
Fundamentally the 5d3 seems to solve all of the problems I had with the 5d2 so the obvious smart money would take that, but at the same time I wonder whether the 1dx would offer me some extra things which were worthwhile.
My only disappointment is the battery life on the 1dx
Look at the Canon site where they list the 1DX features. See what extra it offers and decide wheter you want to pay for them
The headline benefits are probably fps and weatherproofing