Just noticed you said "without" kids. So even more so it's up to what you and your partner are looking to do in the time you are there.

From personal experience, I think Costa Rica would be a better. The people are friendly, there is so much to do for children, and I think the culture is much more eye-opening and inspiring than that of Hawaii. From ziplining, volcanoes, beaches, surfing, scuba diving, rappelling, horse back riding, coffee plantations, rainforests, Costa Rica has a ton of things to keep you occupied.

I also know you can get cheaper flights to Costa Rica than to Hawaii, although I don't know where you are coming from. (I was flying from Detroit to San Jose).

However, you did say only 5 nights which I feel like once you get there you would regret only spending 5 nights there as I did regret only spending 9 nights there in 2011.

Depending on what you had planned on doing in Hawaii (whether you're just looking to lay on the beach for 6 days or if you're looking for more of an adventure and culture change), and how comfortable you are being in a different country would factor into your decision.