If you wind up spending time in Fakarava, consider diving the southern pass off of Tetamanu Village, which is a narrower pass than the northern pass. Hence the sharks are more densely packed. It is not unusual to see over 100 Grey Reef Sharks with a few Black Tips mixed in.
There is a rather rustic resort there - expect mosquitoes (they provide a mosquito net), no hot water, rather dim lights, and the food is so-so. But in terms of the diving and the Edenic setting, it is a "must-do" as long as you're coming this far. I would venture to say that fewer people dive here than exotic and strictly liveaboard destinations like Cocos or Malpelo - so there is that feel of diving a really pristine area where few people set foot and where the coral is in great shape and with lots of fish. It is common to see black tip reef sharks swimming right outside your bungalow. Understand of course that what I am saying here is based on my last trip there in 2006.
You can also do it as a day trip with one of the dive operators that operate closer to the northern pass. There are more choices for accommodations up there.
As for the argument that Fiji offers better diving, in my opinion, it is misleading to talk about "better" diving without taking into account what one is looking to see. "Best diving" for big stuff would be worthless to someone looking for the best macro/muck diving, for example.
Fiji arguably has the best soft corals, and if that is what you're looking for, then for sure it is better than the Tuamotos, because you are going to find mostly hard corals (and in really good shape, I might add) in Tahiti. Fiji has a land-based shark dive in Beqa Lagoon that attracts 7-8 different types of sharks, including Bulls and sometimes Tigers. The liveaboards in Fiji feature a channel dive with Grey Reef Sharks. And Fiji also offers good macro opportunities.
But if it's the sheer number of sharks of different types, nice reefs, lots of fish, and incredibly clear waters that appeal to you, my vote would be for Tahiti, and more specifically, the Tuamotos. And in Tahiti, the French baguettes and croissants are outstanding!