I have worked in both locations so know a lot about them. Fiji has a few big things, mantas and sharks but not guaranteed. Loads of colour, soft coral heaven.
Tahiti on the other hand, the Tuamotu islands to be exact, has loads of sharks, pretty consistent mantas, a resident pod of Bottlenose Dolphins in Rangiroa, massive schools of fish. No colour though as soft corals non existent. Give the Society islands a miss, ie Tahiti, Moorea, Bora Bora- they have some great shark feeds with Big Lemon sharks and mantas at Bora but thats about it. Tuamotus much better diving and much clearer water.
December a great time to go, especially around the full moon as that is when the eagle rays mate in Tiputa pass of Rangiroa, brings in the Great Hammers (thats 16 feet of shark, big enough?)
I would personally suggest the Tahiti Aggressor as you will get 4 atolls instead of one if you go land based, two of the islands Apataki and Toau do not have dive shops and these are the best two atolls for major grey reef shark action, better Rangi because not as deep. When i say major i mean 100+ in a school. Aggressor is a great way to go becuz you get 4 dives per day, food and drinks included. To do the same land based is going to cost the same, hotels in French Poly are not cheap.
And no offense to Triton but the Tuamotus have much better action and more big stuff than Palau/Peleliu, i know that as a fact cuz i worked there for 3 years too.
Both your choices are great, do em both eventually, just a question of where to go first!
