What dive op did you use in both places?
In Fakarava we used Top Dive and were very pleased, this was our first trip there.
In Rangiroa we always go with Raie Manta, as we really love the owner Yves and his wife Livia, they are incredibly knowledgeable, and Yves has been there for decades. He also has a shop in Tikehau and in Rurutu. It is generally close to a private dive if not a private dive.
Also, I agree with Wingy, we love the pensions for accommodations and I prefer to mingle more with the locals there which you miss at the resorts. The most fun polynesian show we ever attended was at Tevahine Dream in Rangiroa and it was the owner and his family. The bungalows are also beautiful and I believe in a few years he plans to build another pension in Tikehau, where his wife owns a motu. Diving in Tikehau, however, is not quite the same as Rangiroa/Fakarava.
For the dive shops, dive spirit was booked when we went and as it turned out, we went with Top Dive by default but were very pleasantly surprised with the quality and the fact that it wasn't a herd of divers, and of the shops on Rangiroa, if we did not go with Yves and Raie Manta, we would likely choose Rangiroa Plongee.