The 1DX is a stunning camera and is top of the pile in most areas, certainly in the Canon model range and marginally trumps most of Nikons too. But that's my point, it only marginally trumps over the 5DIII too. Apart from slightly better AF servo tracking (61 point AF engaged) and double the fps (12 vs 6) then there is little between them. The 1Dx has slightly better iso noise ability, the 5DIII has slightly more resolution. The 5DIII is smaller and lighter...which is often a bonus. The 1Dx has better weather sealing...although I'm finding the 5DIII is the best non-1D series I've used. Both the 5DIII and 1Dx use the same AF array and system, they were jointly developed. Both can use dual cards too. In fact there is more commonality that differences. It's a pity the 5DIII wasn't called the 5Dx. The 5DIII has a silent shutter mode which is currently the quietest DSLR made and it's spookily quiet.
Personally, I'd choose a pair of 5DIII's over a single 1Dx any day.
I have 2 5D3’s and a 1DX… I have had the 1Dx for almost 2 months now and I have to agree with you. The differences are marginal at best. The silent shutter is probably the biggest plus the 5D3 has over the 1DX. More times than not the 1DX is annoyingly attention grabbing and as an avid street photographer, this in combination with the silent shutter is the reason why the 1DX stays home most of the time. If my aim was to impress clients with the 1DX then job done but in reality, images do that. The 5D3 is no slouch in the IQ department. The only times where I can vaguely say I missed an opportunity with the 5D3 is due to FPS and I would have to really stretch to think of 1 or 2 times since owning the 5D3 where this might have happened… Laguna Seca MotoGP, maybe some BIF.
So my 1DX is actually gathering dust most of the time and may possibly get sold. I do commercial/portrait/event photography as a secondary business and justifying the 1DX value (not cost) in comparison to the 5D3 is difficult to say the least.
chilled...i've had the opposite experience. since buying the 1dx a few months ago, my 5d3 is "gathering dust".
to the OP...if you shoot sports or fast action stuff, then the 1dx is a superstar. for everying else, the 5d3 is as good or better.