I have spent time in both countries. It very much depends on what part of French Polynesia you are looking at - the societies - ie Bora Bora, Tahiti, Moorea, Raiatia or the Tuomotos - Rangiroa and Fakarava.
Rangiroa and Fakarava diving will far surpass diving in the societies or diving in Vanuatu.
If you are a fan of wrecks - Vanuatu with the grand old lady Coolidge and Million Dollar Point in Santo, Vila harbour in Efate is full of easy wrecks including a flying boat and the Star of Russia.
Things to do - the tuomotos - dive, eat, sleep, cycle to the village, get a tatu, dive, wait for Arenui supply ship to pull in once a week, wait for cruise ship and locals will bring out pearls and pareos and trinkets....
- The societies - lots of activities for the tourist sector - circle island tours, lagoonarium tours where people have lunch in the shallows with stingrays flopping around on your lap, polynesian feast and polynesian dance shows, ATV tours, Helicopter tours, tours tours tours lol.
- Vanuatu - far less organised tourist activities if you seek them out. You mentioned hiking - Tanna in the Vanuatu archipeligo has the worlds most easily accessible very active Volcano Mt Yasur and I cant stress enough how amazing an experience climbing Yasur is. Tanna is also the home of the cargo cults - last i was there the Prince Phillip Cult were not doing shows but the John Frum/Tom Navy cult do a ceremony at Port Resolution once a week. Also Nakamel village where the people live a traditional life (organise both Mt Yasur and Traditional Village through white grass resort). Also on Tanna, the Tanna Blue Hole, on Espirito Santo Rhi Rhi blue holes are beautiful as is Milennial Cave and Champagne beach when there is no cruise ships in. Malakeula island also has very active volcano hiking and is "the black magic island". Another really interesting thing to see on Santo is the women water singers...its one of those things that you have to see to understand. Pentecost island has land diving which is really something cool to see and theres another little island off Tanna ive forgotten the name of with pretty much nothing there but amazing nature. In Vila pop into the secret garden to look at Sandroing custom and they will also put on a dance.
So, while I would rate diving in the Tuomotos far superior if youre looking for a diving/land based adventure type holiday...surprising myself here..id recommend Vanuatu.