I use an 8" dome with the Sigma. I just think that the overall price difference to have the best lens is negligible in the greater scheme of things.
Personally, I'm very happy with the Sigma. I like it with he Kenko 1.4 too, even though everyone seems to think you can crop better, it gives me a little more reach with animals and is about f3.2.
F2.8 vs. f/4 is very big for fast focusing as well.
I think John's answer is pretty good, if you like the Tokina, go for it. I shot it a lot over the years and liked the results on cropped sensors.
But I was looking to get the best out of the D800 system, and I think the Sigma is better suited and the upgrade price isn't much when you factor selling the older lens.
Edited by JackConnick, 07 March 2013 - 11:17 PM.
Optical Ocean Sales.com Nauticam, Subal, Sea & Sea, Olympus, Ikelite, Zen, Fix, Seacam, Gates, 10Bar, Light & Motion, iTorch/I-DAS & Fantasea Line
Cameras, Housings, Strobes, Arms, Trays & Accessories. System Packages. Photo Expeditions.
Blog & Gallery: Optical Ocean Sales Blog
Flickr Gallerys: Optical Ocean on Flickr