I have a 1dx and a 5D3 so I don't have an angle on this.
They are both great cameras and if you can take good shots with one you will be able to with the other. If I had to choose just one for wildlife it would be the 1Dx, no question or hesitation.
For me the 5D3 has two advantages. First its lighter to carry round so for a days walking about its easier to carry. Secondly the shutter is noticeable quieter, but for me this is no great advantage as I don't find the noise of the shutter a problem even up close. The extra mega pixels is not significant for me so I don't count it as an advantage.
For everything else going that matters the 1Dx is significantly better or has the edge. I'm not going into details or this post will miss my point.
You may or may not find the advantages worth the money. But for me it is definitely worth the cost of the 1Dx. That said I still use the 5D3 for wildlife. I will often take just the 5D3 but if I'm after the best shot I can get for wildlife it will be the 1Dx, probable with the other interested bag with a different length lens on ready.
If you have choose between these cameras you are very lucky as either one will do fine.