Both lenses are great underwater, but probably the main factor in favor of the 16-35 is the fact that it can be used without a dioptre. It is also preferable to use a larger dome like the Zen 230 rather than the smaller 8" domes. Alex Mustard has commented to this effect in his posts and I agree.
Thanks for this, I knew there was a reason one was better. I have the nauticam 8.5" dome. This should be O.K with this lens???