I started a thread regarding 14mm, because I am looking at getting a 14mm rectilinear lens to use UW on a Nikon D200, and was looking for advice from someone who has shot them on a DSLR with small sensor UW.
So I take it you would say that the Nikkor is significantly superior to the Sigma 14mm f2.8 on a DX-sensor for UW shooting?
Also, you mention the need of a 8" or 9" dome. I have a Hugyfot 17.5cm FE dome (7") which works nicely with the Nikkor 10,5FE. Would that cause problems, or do I need a larger dome.
My Sigma on my D2x showed noticably less sharpness well as color fringing at high-contrast areas across the face of the lens. I suspect it would be the same at the lens edges too, but my cropped sensor obviously cuts that out. Given the fringing issues digital can create anyway, I didn't think twice before looking for an alternative lens which would not produce those types of results. The Sigma just doesn't seem to hold up well against the upper edges of resolving power many UW-photo folks are using these days.
So I tested and bought the Nik 14 and, like I commented in an earlier post, this lens seems as if it were designed to be used on crop sensors. The Nik 14 is pretty good at 5.6, but the sweet spot is f8-f11, with f16 again, being pretty good. It isn't a DX lens and Nikon doesn't imply it is designed for digital, but I believe it is their very best WA film lens for use on digital. Plus the only digital prime they make is the 10.5, so unless you are staisfied to use mostly 3.5 to 5.6 zooms the 14mm makes a lot of sense for that apparent 21mm WA lens.
I have used this lens behind Seacam's Fisheye and Wide Ports, but find that it is best behind the Superdome (9"). It seems as if the lighting issues with these super-wide lenses is somewhat controlled or mitigated with a flatter radius dome. Unfortunately I am not familiar with your dome, so I do not known what radius is used for that 7", nor do I know what extension rings, if any, would be required. MFD is 8".
Again, both these lenses in question behave differently on film vs. sensor and topside vs. underwater. I am only speaking to what I get underwater behind a 9" dome on a D2x sensor.