I have an MMIIEX and am happy with it.
The Nikonos is more popular, however.
The arguments fly back and forth, and as near as I can figure, the Nikonos people claim superior optics and better quality photos. I haven't ssen many Nikonos prints that are better than my best ones, and I don't know that anyone has tested them in a lab to compare, but that's the popular perception.
On the other hand, the MMIIEX allows changing lenses underwater in the middle of a dive, which is impossible with the Nikonos.
It's easier to find a used Nikonos on Ebay than it is to find a used MMIIEX, if that's a consideration.
The percentage of Prof. photographers with Nikonos is higher. This may however, be due to Nikonos being out longer and having greater long-term familiarity.
I know this isn't definitive, but it's the best info I've been able to find. Most of us are more limited by our ability as photographers and divers than by our equipment. I certainly am. Whichever you get, practice, practice, practice.
Utah Diving has a photo course online that's a good free starter.
John
Good luck.