Differences between Ikelite ports

I'll be using a Nikon D3200 and Tokina 12-24mm and two or three flashes but I need help deciding on what dome I should purchase. I don't expect to be switching out lenses down the line but it would be nice to have the option.

I was thinking one of these two domes still don't full understand the pro's and cons of each, if the $550 is really worth it, or if someone has a better suggestion:

1)Modular 8-inch Dome Kit with 4.125-inch Lens Extension ($550)

http://www.ikelite.com/accessories/5511.3-dome-kit.html

2) Dome Port for Tokina 12-24mm ($250) http://www.ikelite.com/accessories/5503.51-dome-port.html

About the setup: Due to a number of requirements for my underwater application, the camera will be manually focused to a distance of between ~1ft and 12ft, and all shots during the dive will be taken with the same settings. Photo's will be taken during day and night. uwphotographyguide suggests a dome port with a diopter for the tokina 12-24mm, but I don't know if I need one based on my setup and focus distance. From my reading a diopter is an additional lens that basically clips onto the outside of the dome port? Any comments or advice appreciated. Thanks in advance.

In this case the diopter is inside the housing, on your lens. It's because the lens isn't focusing on what you take a picture of, but a virtual image created by the some port which is closer, and that lens cannot focus that close without a diopter.

If you have to use fixed settings for some requirement then...cheaper port wins!

The cheaper port isn't bad at all.

Would not agree. For rectangular (non fisheye) lenses a larger port diameter will be better. A diopter on the lens might improve sharpness just a bit.

Also remember that the lens will focus on the virtual image, which is typically at about 1,5 times the port diameter. Setting the focus to the expected subject distance before diving will not be in focus if you use the distance scale.

Also curious why you can't use af, you can use back focus to a specific distance underwater and leave it there

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I'm more concerned about using the same exposure settings for every shot. If image quality mattered he wouldn't be using the same settings for every shot.

I have both ports and I take the small one when I travel, or when I'm carrying stage bottles. Less expensive to get scratched. Works pretty well.