I’ve been 2x each to Kauai, Maui, Big Island and Oahu. No reason for first to be one or the other. Big Island has a different feel but we like it.
If you do Oahu, I’d stay on North Shore vs Waikiki. Kauai is good in Poipu or North Shore. I liked north shore best but VRBO/Air BnB makes it harder to not stay in hotel/condo areas. Maui is great too…need to do Road to Hana to get Hawaii feel outside of resort areas.