I think this is a question on the minds of many people: What's best for video at around US$1000?
- Blackmagic Pocket Cinema Camera
- Panasonic GH3
- Nikon D7100
The question is more complicated, though. Do you want to be a film maker or just make videos?
The Blackmagic Pocket Cinema Camera should be great for the film maker in training, as well as those already in the business. However, it is not a stills camera. You can extract a still frame from your video files but it's just not a camera body for stills photography. The sensor is optimized for great video capture but it's a bit small for stills, especially given that it's using micro Four-Thirds lenses including those lovely Carl Zeiss Cine lenses up to US$20,000 each.
If you're still with me, you probably don't want to invest in cine lenses or make films.
Your video choice is easy from here: the Panasonic GH3 is much better at video than the D7100. The GH3 is likely as good as the Canon 5DMkIII but even if it's not, you can buy almost 3 GH3 bodies for the price of a Canon 5DMkIII.
I wouldn't say that the available lenses for micro Four-Thirds are amazing (except for the very high priced fixed focal length lenses), I would say that they work very well for video. What's more, you can use Four-Thirds lenses, Leica M lenses, Leica R lenses, and more with adapters. You'll find the Olympus ZD 7-14mm f/4.0, ZD 14-35mm f/2.0, 35-100mm f/2.0, and 90-250mm f/2.8 some of the best lenses--ever.
Given that the video and film industry grew bits and pieces to make the Panasonic GH2 body work for them, it's amazing to see them adapt to the GH3.
As for the Nikon D7100--it's the finest APS-C sized sensor-based camera body out there right now for stills. It's up-to-date in 2013 with some great pieces from the D300s but with a great sensor and missing an anti-aliasing/low pass filter for better sharpness at the risk of moiré (like mwah-ray, American English speakers) patterns. Professional raw development software should help you lessen any moiré which might intrude into your photos. In many ways, the D7100 is better than the budget D600, which is also closely based on the D7000 but without the bits from the D300s. Sure, the FX size (135 format) sensor is bigger but the D600 is a budget camera body. Plus, because of the sensor size, that 70-200mm is just 70-200mm but you get that extra boost in the D7100 as it becomes an effective 105-300mm lens. The only real drawback you'll see is in the buffer size. When they increased the pixel count, they should have used a buffer size that was much bigger. Giving it most of the D300s' pieces but leaving the buffer size too small was not the greatest idea, especially without a D400.
Update: The D600/D610 are available are based on the D7000, which was sufficient but not good at video, and certainly no better than the D7100. Equally, the Canon 70D and below have been updated but they're only really competition for the Nikon D7100 and below. The Panasonic GH3 would easily be a better choice, especially with the articulated rear display and multiple file formats and bit rates, as would the "hacked" firmware version of the GH2.
Update 2014.03.21: This situation hasn't changed completely. The Pocket Cinema Camera had a rough start but seems to be fine now. The D7100 is still available without change, but the GH3 is being replaced with the GH4. Does 4K video change the argument between buying the Pocket Cinema Camera and the GH4? Yes. US$1699.99 vs US$999.99 gets you 4K and 4K Cinema video in a powerful package made for almost any use considered. Panasonic even added motion picture industry-related terminology to the GH4 menus, courtesy of the AG-AF100/AF105.