My family has found that Forbidden Island is far more fun than Forbidden Desert. (Obv, ymmv). We just don't feel the need to have both of them, so one has been relocated to the grandparents house.
I have both, I liked Forbidden Island more, and my GF and her son both like Forbidden Desert more. I think the main difference is Desert involves a lot more luck, you can't really plan ahead much because the game could change considerably on one play. So Island would appeal to people that like to plan ahead, and Desert would appeal more to people who like chaos and drama.