Bula!
You have picked some nice spots.
Kadavu - 3* have a look at
Matava and for a bit more up market,
MaiDive on Ono which is just north of Kadavu but in the same reef system. Do bear in mind you will have to fly there and back from either Nadi or Suva (Nausori).
On Taveuni, the resorts mentioned above are lovely but have a look at
Garden Island resort as well, or if you want the same dives but a bargain, look at
Dolphin Bay Divers Retreat.
Taveuni offers lots of inland walks and cultural opportunities. It is a little more accessible and you don't have to fly there as there is a 3 times a week ferry service from Suva.
Savusavu is great for access to the Namena Marine reserve. You can stay in the town for a bargain price and dive with
Korosun dive on a daily basis. They don't go to Namena every day though.
Tui Tai is a one off kind of trip. Rather expensive in my opinion, but you will get to see some of Fiji's less accessible reefs, and have a massage afterwards.
If you wanted the best of Fiji diving on a liveaboard, have a look at
Naia cruises. Better value for divers than Tui Tai.
One other thought is a private charter dive / sail available from Suva and you can dive Kadavu, Beqa - including the Shark dive if you wanted, and sail in luxury at the same time. Let me know if that's of interest and I will send you details by PM.
As for an itinerary, I would fly to Kadavu fro Nadi, then fly back to Suva and get the overnight ferry (in a cabin) to Savusavu, Ferry to Taveuni - couple of options there bu the Grace ferry and bus in very cool, if a little bumpy, and then fly back to Nadi from Taveuni.
Or Fly to Sausavu from Nadi, ferry to Taveuni, ferry to Suva and 5 nights sail dive charter around Kadavu.
Easy to do in 2 weeks with a bit of planning.
I can organise bookings for you if you wanted any local logistics assistance

Have a great trip.
Greg