What's the difference between a 7mm and an 8mm fisheye lens?

Both for Nikon, and both say 180-degree field of view, so where's the 1mm difference?

8mm: http://rokinon.com/product.php?id=8
7mm: http://www.cameta.com/Vivitar-Series-1-7mm-f-3-5-Fisheye-Manual-Focus-Lens-for-Nikon-Cameras-42593.cfm

There's virtually no real difference, and it depends exactly upon how the measurements are taken. The 180 degree f.o.v. Is an approximation as is the focal length, so as an example a 7.4mm fisheye may be called an 8mm by some manufacturers and a 7mm by others.

But both lenses appear to have an identical specification and near identical weight, and both brands are badge-engineered models: another manufacturer makes them and applies external details to suit the brand. There simply aren't that many manufacturers of such a specialist lens, so it's highly likely that they may be the same model under the skin. In that case it would be common to slightly massage some specs to make them seem like different models.

Very few retailers will have them in stock so you're unlikely to be able to try before buying, so just go by whichever offers the best deal in terms of price, warranty and supplier trustworthiness. Vivitar are a far better known brand than Rokinon (not that it makes them any better, they've stuck their name on some really poor lenses at times), so will likely have a better resale value. Resale value is important as a 7-8mm fisheye is something which is fun to start with but rapidly becomes really boring. For that reason alone you may wish to look for used examples which will be far cheaper.