Thanks for the reply. It seems that there about a $200-$300 difference between the two lenses. Are both lenses going to do the same thing in the end or is one lens better than another for the 360's? If you were going to make a purchase, which one would you buy?
I don't have fullframe sensor camera - so for my Nikon cropped sensor camera I would buy the lens designed for crop sensor cameras - namely the Tokina 10-17mm zoom FE.
Except that I already have the Nikkor 10.5mm FE - the equivalent of the Canon 15mm FE on the Canon 5D - which does that job for me on my Nikon D40 cropped sensor camera.
If I had the fullframe sensor Canon 5D I think I would buy the Canon 15mm f2.8 FE I think. But that's because I'd be happy to take Z+6-around+N shots for 360x180 coverage; perhaps that's more shots than you wish to shoot for sphericals?
Being able to shoot a spherical in just 3 or 4 shots - which I understand you can do with a 'shaved' Tokina 10-17mm zoom FE on the Canon 5D - is useful if you plan to shoot panos where there is a lot of movement in the scene (I bought the Sigma 8mm f3.5 FE for doing that with my cropped sensor camera).
But 'shaving' the sunshade of a Tokina 10-17mm zoom FE so it will work usefully on a fullframe camera like the Canon 5D is not a task for the faint hearted! I believe thee are some people who will do this as a service for a fee if you wish to consider it.
Others may have a different take on your question.