Hi: I have dove both the Cook Islands and all the major dive areas in Fiji. If you want good diving you definately should go to Fiji. I also recommend Wananavu with Kai Viti Divers......the diving is really incredible, probably the best in Fiji and the package pricing right now is really, really good. Wananavu Resort is beautiful and a great value....but we also have a great price with a budget hotel if you want to keep the price really low. I also recommend the diving from Taveuni and my favorite resort in the North is also Matangi. Matangi is a deluxe/luxury property and has a special rate that offers 2 free nights with a 7 night stay.....this is a really great deal. I also like the diving on the Astrolab Reef from Kadavu Island, (South of the main island), this has great variety and very pristine.
If you wanted to visit the Cook Islands because of the great beaches and beautiful lagoons.....you would be going to the right reason. The Cook Islands are really beautiful.....I love Aitutaki, I think it is probably one of the most beautiful spots in the world. The diving is okay.....but definately not as good as Fiji.
If you want pictures or just to chat about the diving in both places feel free to ask.
Hope you have a great trip.
Mary Jane