Sigma make the lens at 180 degrees (but this is only true on a full sized sensor) I.E: A film/analogue SLR camera or a Canon 5D, 1Ds or Kodak full sized sensor DSLR. All other DSLR's have a multiplying crop factor with Nikon being 1.5x and Canon being 1.6x. The Canon 1D (not 1Ds) has a crop multiplying factor of 1.3. Think of these as default zoom.
You can see this clearly from the samples supplied. Notice the full frame sensor (Canon 5D and Canon 1Ds) is full circular 180 degrees.
Now shooting with these higher professional cameras that give a full circular image allows you to stitch as little as 3 images which make life easier but! You will also notice that much of the sensor is not used and you waste a lot of pixels to "black" nothing area. The offset to this is that the professional full size sensor cameras have much larger Megapixel sensor that compensates.
Really this is why using a full frame fisheye (15mm/16mm) is better suited for higher resolution shots using the Canon 5D/1Ds as you don't waste those precious pixels. Or you can adapt a Nikon/Nikkor 10.5mm Full Frame Fisheye to the professional Canon body.
I think the real answer is possible to use the new Tokina 10-17mm zoom fisheye on these cameras to get the best of both worlds.
You will get some saying you are better off with a small crop sensor and shooting more images. (That is hard to argue!) But the 5D for me is still the way to go because it can be used for so many other options and "All" SLR lenses work the way they were intended on a 35mm camera.
Also understand the Nikon FC-E8/9 are not always 185 degrees. It depends on what camera they are fitted too. On some they are less than 180 degrees and on others more than 190 degrees.
Finally, I have owned many Nikon's and many Canon's. I absolutely love my 5D but only slightly over my previous love the Canon 10D. I'm sure the 20D and 30D along with the 300D, 350D, 400D are all very good for their owners as well.
Hard to find a bad word about the Nikon DSLR range either.