Been to both and definitely would go for NZ. We went on our honeymoon to NZ & Australia in early December '93 before the weather became too hot. As others have said you can hire a camper van or stay in relatively inexpensive hostels and eat a lot more cheaply than on the Hawaiian Islands. IMO Hawaii is a disneyfied Polynesia, with the exception of The Big Island, and seems to be very commercial with timeshare sellers trying to talk you into buying their goods if you are a US citizen. The seas are warmer and the beaches are great. In New Zealand the wildlife is incredible with thermal springs and walkable volcanoes as well as amazing glaciers and heli hiking, whale watching and penguin spotting. The Maori sites are numerous and the Hangi's feel less staged and you can immerse yourself more in the Polynesian culture. Have you considered doing New Zealand with The Cook Islands as a stopover to get the best of both worlds?