When I was in the Taveuni area (land based diving btw), soft corals were the big attraction and compared to the rest of Fiji, are known for it. That doesnt mean you wont see soft corals around Viti Levu. We did see white tip reef sharks but as singles and not in great numbers. Larger fish would be seen in the Somosomo straits and around Yellow wall and Noels wall. We were told that occasionally mantas could be seen at these sites but we did not see any. We dived 3 days diving around Yellow and Noels wall and one day on the Great White wall, Rainbow passage and Jacks Place. The larger pelagic encounters in the Taveuni area were described as sporadic and depends on season. We did see quite few turtles especially around Qamea island.
I dont recall reports of hammerheads schools or solo. It wasnt mentioned as a draw for diving there. However huge schools of smaller fish occurred quite frequently at most sites especially walls though these were the areas we were told to keep an eye out for sharks or other large pelagics like mantas but we never saw the latter.
It is my understanding that greater numbers of large fish/sharks occur around the main island of Viti Levu. There are planned shark dives (Beqa Lagoon for example). I dont know about mantas in the area.
If hiking is in the agenda and staying in the Taveuni area, then Bouma National Park with the Tovoro falls is excellent. Taveuni can be a bit wetter than other Fiji locations. It doesnt rain constantly but is more wet in April but there are wetter months. We had sunny days most of the time though.
My wife and I stayed at Matangi. This is a higher priced place and I have come to the conclusion that I paid for seclusion. I think max occupancy is 13 couples and was not close to full when we were there in the month of April. Its a fairly small property on a small island (no one else but them). If I went back to Fiji, I would stay on Taveuni (for less $) but it is farther from my two favorites sites (all time) Yellow and Noels walls.
I have not dived the Maldives but it is on the list. We have looked into it but it is easier for us to get to Fiji and even easier for you on the West coast.