Both are SLRs, both are a few years old. But the 5D has a 12.7 megapixel, full-frame chip, where the Sigma produces a mere 4.6 megapixels. Why bother even comparing them?
The reason is that the Sigma has a Foveon sensor rather than a CCD or CMOS. This sensor can capture distinct R, G and B light information at every pixel. The 5D's CMOS chip can record luminance at every pixel, and uses the common Bayer pattern of color filters at the photosites to capture RGB color, intermingling color fidelity and spatial resolution in a way that must be decoded by software using some math, some compromises, and some guesswork.
One way of looking at this is that the 5D spends three of its 12.7 million pixels to accomplish what the Sigma achieves in only one. If the 5D records 12.7 million tiny little light records per image, then the Sigma records 13.8 million (4.6 x the unique R, G and B records per pixel).
But that's not entirely fair. Our eyes tend to perceive detail more in luminance than in color, and the 5D is recording much more luminance information that the Sigma. For black and white photography, the 5D truly is a 12.7 megapixel camera and the SD14 truly is a 4.6 megapixel shooter.
So the truth lies somewhere in between—the SD14 is neither the 5D's equal in resolution nor is it possessed of one third the pixel count.
In one very real way the RED One is a 4K camera. It creates 4K images that look damn good.
And in another equally real way, the RED One is not a "true" 4K camera, as each of the 4K's worth of pixels it creates for each frame is interpolated (from compressed Bayer data at that).
And that's probably just fine—a topic for another day.