I'm a cinematographer/photographer. I take sports videos and I need a new DSLR. I heard that Nikon is horrible at taking videos because of the format, or for other reason, I forgot.
Is this real?
I also heard that Canon is the best one for videos. Why is that? Is this a myth too, or a fact?
What DSLR are you using now? That will dictate the wisest use of your resources. Also, what is more important? Sports videos, cinematography, or photography? What kinds of sports videos (highlight reels or full events)? What kind of cinematography? What kind of photography?
I haven't been in the video game since I sold my DVX100A long ago, but my friend is a cinematographer and we both like to keep up on all the new technology.
If I were to get back into dramatic scripted work, I would invest in a Blackmagic Cinema Camera, which is available in an EF or a Micro 4/3 mount. I would get the EF mount because all my lenses are EF mount. While the Micro 4/3 version will accept virtually any lens with an adapter (including PL mount), you cannot electronically control the aperture of EF lenses if you choose to use one.
Why would I pick that over a DSLR? Despite all its ergonomic shortcomings (allegedly; I have never handled one), it shoots RAW video. Just watch these videos:
http://vimeo.com/49875510
http://vimeo.com/48085024
You simply cannot get that latitude or resolution with a DSLR, save the 1DC, which is apparently only a 1DX with a firmware update. And, it's cheaper than a 1DC, 1DX, and 5Diii for that matter.
Of course, this camera has many drawbacks too. You can't get everything for $3,000. It is not the ideal camera with which to shoot a sports event. So if that is more important to you, I would probably look at a camera with real autofocus, like the Sony FS100 or FS700.
If photography is your aim, obviously you must go for a DSLR. It should be dependent on how much you have invested in lenses for a particular system and the demands of the work you are shooting. Just make sure to research 1.) which can autofocus while recording 2.) which can adjust settings manually while recording 3.) which have or don't have a 12 minute recording limitation. I will mention that I can only imagine shooting a full sports event with full-frame sensor would be a pain in the ass to focus!
Also now that I am coming to a conclusion I realize I have completely side-steped your original question which was whether or not Nikon is horrible at videos. (Sorry, I've had 4 martinis.) I'm assuming you are implying not about ergonomics but rather image quality. I have to say that it's not horrible at all because if you do a quick search on Vimeo, the quality of those cameras are fantastic. Check out Chase Jarvis's D7000 video. The D800 shoots clean 1080i uncompressed out of the HDMI port, so if you have an external recorder, it can look really good, except for the moire issue. (The reason the D800 is so sharp is because it has a really weak anti-aliasing filter, which is great for photos but not as great for video.) Compared to a 5Diii I would imagine it looks pretty similar but I can't recall as it's been a while since I've seen comparisons. Apparently the D4's video is not as good as the D800's. The 5Diii can shoot at a higher ISO before it gets noisy. Bottom line is that they're all pretty similar and it depends on your needs and the needs of your clients.
Hard to suggest anything specific without knowing exactly what you plan to shoot, what you currently own, and what your budget is. Give us these details and perhaps we can offer some suggestions!