This is a frequently asked question. The short answer is that there is no difference.
Rarotonga and Tahiti are geographically right next to each other so the sea conditions are identical. Furthermore, the oyster (Pinctada margaritifera) that is used to cultivate the pearls is exactly the same species in both countries and the farming techniques used are alike. Lastly, the ratio of low quality versus high quality pearls is proportionate.