Ironically, we've been three times to Fiji in the past 15 years (twice at Turtle Island--which we do not recommend--and once at Vatulele Island--now closed)...and will only be making our first trip to Tahiti next April.

Fiji is almost as easy to reach as Tahiti--the flight is just a bit longer (10-11 hrs on Fiji Air vs 8.5 hrs on AF/TN from LAX). Usually, the LAX flight to Fiji is cheaper than that to Tahiti, even in business class.

Fiji seems less over-developed compared with all that I've seen and read about Tahit. There also are many private island resorts in Fiji compared to Tahiti. The OW bungalow phenomenon isn't a factor in Fiji, thankfully...and the prices usually reflect that! Overall, I think you get more for your money in Fiji, even/especially when staying at the private islands. (We don't recommend Turtle Island due to its horribly homophobic owner and staff, however.)

While we cannot recommend Turtle, I can say that one of the most amazing beaches we've ever experienced was there...and that the Fijian islands have the softest sand we've ever had. So there are good reasons to go over Tahiti IMO.