All of these lenses are the very same thing with a different name on them. They are "all" Samyang f/3.5 8mm Fisheye. Forget the rubbish about 6.5mm, 7mm etc they are all 8mm.

http://www.amazon.com/VIVITAR-Vivtar-Fisheye-Digital-Cameras/dp/B002H06U50/ref=sr_1_175?ie=UTF8&qid=1338392648&sr=8-175

http://www.amazon.com/Rokinon-FE8M-C-F3-5-Fisheye-Canon/dp/B002LTXQUE/ref=sr_1_2?ie=UTF8&qid=1338392357&sr=8-2

http://www.amazon.com/Bower-SLY358C-Ultra-Wide-Angle-Fisheye/dp/B00317H7Q4/ref=sr_1_19?ie=UTF8&qid=1338392421&sr=8-19

http://www.amazon.com/Opteka-Aspherical-Fisheye-Digital-Cameras/dp/B002KLY9I6/ref=sr_1_69?ie=UTF8&qid=1338392508&sr=8-69

http://www.amazon.com/Bell-Howell-Aspherical-Fisheye-Cameras/dp/B001IJAF4S/ref=sr_1_87?ie=UTF8&qid=1338392534&sr=8-87

All the above are the same thing! Buy the cheapest one.

This one is a 14mm Samyang

http://www.amazon.com/Rokinon-FE14M-C-Ultra-Canon-Black/dp/B003VSGQPG/ref=sr_1_64?ie=UTF8&qid=1338392477&sr=8-64

These ones are NOT fisheye's so I'm not sure why you have them listed?

http://www.amazon.com/Sigma-18-50mm-2-8-4-5-Aspherical-Digital/dp/B002DGXUQK/ref=sr_1_86?ie=UTF8&qid=1338392534&sr=8-86

http://www.amazon.com/Rokinon-85M-C-Aspherical-Canon-Black/dp/B0025EWXEU/ref=sr_1_123?ie=UTF8&qid=1338392582&sr=8-123

The upshot is, the Samyang 8mm is a fine lens and suitable for your needs. It will require a minimum of 4 rotational shots and a Zenith/Up shot to cover the 360x180 sphere. All that said, there are the odd bad one that is released so make sure whichever you buy you have a good return/swap-over policy.

These lenses are 100% manual. Meaning they have no electrical contact with the camera body. You have to manually set the Aperture and manually set the focus. (Which you should do with all Fisheye anyway