Tahiti would be my choice. My friend went there last year for their honeymoon, and got one of those Bungalows out in the ocean with the glass floors, it was 1 grand a nite, they were very happy and the pictures look worth it.
I recommend you Tahiti. I am sorry that I have never visited Fiji, but Tahiti is the heaven on earth. Hotels over there are excellent, and the beach and weather over there are superb. I strongly recomment Tahiti