Tahiti or Fiji for stopover?

Hi all. We are leaving for a RTW trip in Jan 2015 and can stopover for cheap in Tahiti or Fiji on the way to NZ. Not really sure which of the 2 to pick. I have never been to the Pacific so would love some classic beach paradise but we would like to stick to a budget. We would also like some area with local hikes, waterfalls, etc. Any opinions on which is better?

Any opinions on which is better?

I have never been to Tahiti, so I cannot make a direct comparison with Fiji. However, if you want to stick to a budget, then Fiji is probably the least expensive place to visit. There is also enough of a range of tourists that there are even 'resorts' that cater to the low-budget backpacker types. The multi-star holiday tourists tend to stay in their resorts, leaving huge areas of Fiji with very little tourism.

The island of Taveuni is a good place to go in Fiji if you want some nature hikes, waterfalls, etc. For the classic beach paradise idea, you'd want to head to a small lagoon island resort somewhere. The Yasawas are very popular, but there are quieter, less expensive spots elsewhere.

Ideally, you'd want a minimum of 2 weeks for this.

I was in the same situation as you, and I met quite a few others with a RtW ticket, slotting in Fiji. Low cost was the main consideration for just about everyone (Nadi also being a transport hub ..). All else being equal (meaning, no personal interest in a particular country destination, but still want a S Pacific stop) you can make your money go further on Fiji.

Can only second the above.

Fiji is definitely much cheaper while Tahiti could be more scenic.
Another factor in favour of Fiji could be that it makes an excellent hub to explore other countries in the region.