I assume you mean 1Dx Mark II or a Nikon D5.You are in a Canon forum. I think most of the replies will point to a Canon body.
The camera specs that you would want for wildlife photography can be very different from the camera specs you would desire for movie making. I feel that both bodies you mentioned are aimed mostly at the pro sports photographer. Either body can do any task very well, though. I think other factors should be considered before making a final choice of camera body.
The desired lens performance can vary just as widely as the camera specs. I think Canon has better lenses than Nikon. Not only do Canon lenses perform better, they are lest costly than the Nikon equivalents Canon also makes professional video camera and broadcast quality lenses, while Nikon does not. Canon lenses can be found everywhere, from scientific and medical equipment to EF mount security cameras.
When I go out looking to shoot birds, I want as much focal length that I can get. I do not have the budget for an 800mm prime lens, I buy lest costly zooms and use an APS-C sensor body, a 7D Mark II. I suppose the Nikon equivalent would be a D500.
I pair the 7D2 with an EF 100-400mm f/4.5-5.6L IS II USM. I have a 1.4x extender, but it reduces the 7D2 to just one AF point, which acts pretty sluggish compared to without the extender. The 80D would have 27 AF points with an extender, and none of the AF points take a performance hit. I suspect the just released 90D performs just as well.
The biggest drawback with using the 7D2 is that it focuses very well in bright light, but not so well in low light, where it also tends to get noisy with higher ISO. For this reason, I leave the 7D2 at home ono overcast days, or for shooting Friday Night Lights. I will bring a 6D2, which has the same excellent AF focusing system as the 80D.
As far as video goes, I would want to use a camera designed for capturing video. Canon makes professional grade camcorders, which are popular for ENG applications. Once again, this is a camera market where Nikon has no presence. If you want a professional video camera with interchangeable lenses, then look at the Cinema EOS line of cameras.
I bought the entry level C100 a few years ago when the priced dropped with the release of the C100 Mark II. It captures full HD video with C-Log, and include Dual Pixel AF. It does the jobs that I need. If you need 4K video, then take a look at the C200.
All Cinema EOS include Canon’s C-Log, which is not available in the 1Dx Mark II. The EOS 5D Mark IV has C-Log available as an option. I have never heard of anyone making movies using Nikon gear. Wedding videos, yes. Cinema quality movies, never.
If I had to make a recommendation, I would suggest the 1D Mark II over the Nikon D5 for what you want to do. Another option would be a 5D4 or Cinema EOS body for video, and a 7D2 or a 90D for wildlife photography.
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