WE ARE IN THE BEGINNING STAGES OF PLANNING OUR TRIP TO AUSTRALIA. WE WILL BE GOING FOR 3 WEEKS THIS SEPTEMBER. WE ALSO WOULD LIKE TO GO TO EITHER FIJI OR NEW ZELAND AND WE ARE HAVING A HARD TIME IN DECIDING WHICH ONE TO GO TO. ANY SUGGESTIONS WOULD BE APPRECIATED. THANKS.

More information is needed to give a focused reply to your question.
It totally depends on how much time you have in your second destination and what your interests are.

Fiji can be a great 3-4 day stopover for lounging on the beach and snorkeling after a hectic trip in Australia. But it takes at least 2 weeks to see much of anything in NZ, which is more jampacked with amazing stuff to do and see than any other country I have ever visited.

Fiji and NZ are completely different destinations, either of which could be great for you, depending on what you are looking for.

Fiji and NZ are not comparable. For a start Fiji Islands are in the tropics and NZ climate ranges from temperate to alpine - how much time are you allocating?

Yes, I agree with others that Fiji and NZ are very different choices. Fiji was a stopover for me on two trips to OZ, on the way back home. Two to three days was just perfect. I stopped in NZ on the way to OZ on another trip down under, and spent a week on the north island. I saw the Bay of Islands area, Auckland, and Rotorua. It was a great trip! It is a do-able. NZ is a wonderful country to visit. I'd prefer that over Fiji.

Sorry, I meant to add that if you only have a few days and like beaches, snorkelling etc then make it Fiji. Save New Zealand for another trip - it deserves at least 3 weeks on its own.

JUST A LITTLE MORE INFO. WE WILL BE FLYING IN ON QUANTAS FROM LAX, THEY OFFER YOU A STOPOVER IN FIJI OR NZ. ON THE RETURN FLIGHT YOU CAN ONLY STOP IN NZ. WE HAVE ABOUT 4-6 DAYS TO SPEND IN EITHER LOCATION. IF WE GO TO FIJI IT WILL BE EARLY SEPT OR IF WE OPT FOR NZ IT WILL BE THE END OF SEPT. WE ENJOY SIGHTSEEING AND WATER ACTIVITIES. THANK-YOU FOR YOUR RESPONSES.