Is Nikon D750 a good choice vs. Canon 5D Mark III for avid Videographer?Edit
You should consider other options than Nikon/Canon. Even if they have proven DSLR can do really good video, thanks to 5D Mark II/III and Magic Lantern, they do not compete well against Panasonic and Sony.

If you compare 5D Mark III against a Sony A7S, you're going to have a much better video quality on A7S, thanks to its far superior sensor. Much higher dynamic range, better low noise performance, 4K, and so on. Watch the 5DIII vs A7S:


                
            

The mind-blowing low noise performance on A7S:

https://vimeo.com/99893160

Again, 5D III is left behind Panasonic GH4. The last one is far less expensive, and offer 4K resolution. Even if low noise is not really well performed on it, because of its 4/3 sensor, it's still a good competitor regarding of its price.

Nikon and Canon seem to be left behind nowadays. They provide the best DSLR for stills, but for video, you can find better, and often with less money.

Wake up Canon!