Bathys Topdive are the ones to go with. If you dive on Nitrox, that will enable you to get an extra days diving in at each atoll.

I've only dived at Tikehau and Manihi. Tikehau has a long boat trip from Pearl (about 45 minutes each way) but the trip is worth it.

Mostly drift diving in the atolls, as all the best diving is done in the passes. That's where all of the sharks hang out. You'll also get good photos of Mantas in the several cleaning stations.

As far as pelagics are concerned - you'll get ALOT of that on any atoll. The main drawcards are the hammerheads, mantas and lemon sharks. Plus a truckload of reef sharks - you'll get sick of photographing them.