Spent 10 days in raro and aitutaki last month.
Snorkelling is good all around raro inside the reef. Lots of nice sandy beaches and easy access. The most fish (and biggest) were in a place called 'fruits of Rarotonga' which is inside a marine reserve on the southern side of the island. Easy snorkelling - good for kids.
Only did scuba diving on Aitutaki. Went out with "Bubbles Below" and hired all the gear. No need for wetsuit which makes a pleasant change from here. Even at 30 m I was still warm. Got photos of large green turtles (4 feet shells), massive Napolean wrasse about 4 feet long and 2 feet high, moray eels, white tip reef sharks. On the day I dived the visibiity was only about 30m - apparently the wind direction/swell murked it up a bit - normally it is 40-50m. The bubbles crowd were really good - easygoing, friendly and knowledgable.
Haven't been to Fiji but enjoyed Aitutaki in comparison to Hawaii, Green Island and Poor knights. Enjoy ...