My main uses for getting a DSLR for Video would be the following:
- HDR (Video/Audio)
- Slow Motion
- Excellent Live View
- Good RAW Support
Any other pro-features for videography.
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:
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!
No, the D750 is not a good alternative for an avid videographer, at least not if you want the advantages Magic Lantern offers. ML isn't available for Nikon. The reason has nothing to do with the technical advantages of either platform, you can get models of camera that are fairly close in that regard.
The problem is that Nikon does not have anything like Magic Lantern. The majority of your high end professional video features, such as RAW video and HDR video, are not actually supported by either make on it's own. They are added by a third party firmware hack called Magic Lantern and it is only available on select models of Canon camera.
If you aren't looking to use a hack, the D750 may make a decent enough option (I haven't looked at it that closely since I already have a 5D Mark iii), however it won't have many of the features you are looking for, just like a stock 5D Mark iii wouldn't.
Other key features you would be missing are things like focus peaking and adjustable zebras. The scope support is also much less advanced, though either camera can support uncompressed HDMI output which means you could use an external scope and even external focus peaking if you had the right hardware, but that's more to buy and offsets a lot of your cost savings.
Update: I did some more reading on the D750 due to some of the comments and I still stand by the majority of my comparison. This is a low light comparison of the two cameras.
Note in the high ISO video that the 5D Mark iii absolutely destroys the low light performance of the D750 on video. The one thing that is noteworthy about the video is that one native feature the D750 offers is flat color profile. On the clip where it shows a flat profile, the Nikon will appear better because it isn't using any shadow detail, however the color depth will suffer in that mode.
You can do flat color with 5D Mark iii as well, but it requires either some careful custom adjustment or use of ML to do so. Additionally, the 5D Mark iii with ML offers true raw shooting which blows flat profile shooting out of the water since you maintain color depth while still having the ability to adjust color and luminance response.
The dynamic range available in video shot with ML is much higher as it can be set to change ISO every other frame and then, with some post processing, do an HDR blend for each frame.
The timelapse feature on the D750 may be nice to have, but it saves relatively low quality and ML provides this feature on the 5D Mark iii with full raw support and extensive options.
Note however, none of that is to say that the D750 isn't an excellent camera. It's easily one of the best, if not the best video feature set that Nikon has made yet. It's a solid camera, but it can't hold a candle to the capabilities of the 5D Mark iii in the video space when running ML and still has some significant short comings even if you don't run a 5D Mark iii on ML.
The camera is not the only issue here. If you have Nikon lenses you can get great footage with a D750. I am sending my Sony A7S back to BH because of horrible focus issues in low light. If a scene is not well lit there are issues no matter what camera is used. You need a good prime lens to shoot in low light, not high ISO.
I use Nikon SLRs among a suite of other video cameras for professional corporate videography. The latest full-frame Nikons have proven to be good solutions for a higher-end look. As with all SLRs, they are far from the convenience of a typical, all-in-one ENG type camera and as with all SLRs their video capture quality is lacking. My solution if the Nikon is my A camera is to use a separate pre-amplifier for audio. The other technique is to use an off-camera video recorder to capture off the clean HDMI port. Other, in-camera, methods such as Black Magic's firmware might work for some producers but I prefer a high bit-rate file such as a real ProRes 422 or HQ file which produces cleaner and easier to work with images in post. Nikon's latest flat picture profile, while not S-log, has been a joy to use as it stands up to color correction. In shoots where I have to deliver almost immediately, other profiles work nicely as well. In the end, I've been happier with the results from the latest Nikon SLRs over the files out of Canon's MkIII. Nikon's files rival Sony's.