Hi Linder,
Storker linked to my thread and many of those shots were at 12mm. Remember that 12mm is only "semi-wide" underwater. You will get a little more coverage with a dome, but nothing like a dedicated true wide angle like the 9-18. You can still get nice scenic shots at 12mm even behind the flat port, but not that ultra-wide look. This goes back to Storker's comments about considering what you shoot.
If you are a generalist who might encounter anything on a dive from tiny macro to scenic to bigger fish or mammals (not whales or whale sharks, but things like sharks, turtles, dolphins, large groupers, eels, etc), then the only choice, to start, is the 12-50 in the dedicated port. Nothing comes close to its all-around flexibility.
If you are mainly into macro but want flexibility for scenics or larger subjects, the 12-50 in the dedicated port is also a great option, as the 60 macro works perfectly in that port as well and gives you true ultra-macro for macro dedicated dives (think Lembeh), but also the flexibility of the "all around" lens for dives where you want that.
If you are mainly into wide scenics and huge animals, then the best start is the 9-18 and 60mm in the dome. Here, you lose a little on the macro end because the dome will not let you get to the closest focusing distance without being too close to the subject, but you can still get very good macro results.
Either way, you are buying one port and two lenses. The latter option will probably cost you a little more.
Most cost-effective to start is the 12-50 in the dedicated port and then add other things as you decide what you like. I can tell you that the macro on the 12-50 is so good that there is no hurry at all to add the 60mm.
