In the below video, Tim Shoebridge compares the native 4K output of the Sony FS5 and Samsung NX1 along with the footage when downsized to 1080p.
Shoebridge explains in the video's description:
Both cameras were fitted with the exact same lens: a Samyang 35mm f/1.5 cine lens, Nikon fit," "The NX1 was shot with its flattest profile at 1600 ISO, 1/50th shutter speed and aperture set to f/5.6. The Sony FS5 was shot in SLOG3 (picture profile #8) at 3200 ISO, 1/50th shutter speed and the same f/5.6 aperture.
The footage was edited in the latest 2015 version of Premiere Pro which has native support for the Samsung NX1's h265 codec video files. No post-processing was performed on those file at all. Everything, including exposure, white balance and contrast curve is exactly as was shot.
The Sony FS5 SLOG3 footage needed post-processing in order to bring it to a similar colour and overall contrast to the NX1 footage. However processing was kept to a minimum and no sharpening was applied.
The difference in field of view between cameras is very unfortunate but was non intentional on my part. Both cameras have APS-C sensors but they must use different portions of their sensors from which to generate video footage.
Regardless of field-of-view difference, I think the results speak for themselves. The NX1 out-shines the FS5 in every way. And the FS5 footage is extremely similar in native 1080p compared to downsampled 4K UHD, at least to my eye.
He recommends downloading the file to see the true comparison which you can do here.