Last year about this time of July we dove off Kona the Big Island to see the Mantas. We took two days of diving on the Big Island to improve our chances. We were quite dissapointed not to see them. The diving in Hawaii is not near as good as the Carribean, although it depends where in the Carribean you've been. Hawaii is about equal to Freeport Bahamas divng, not allot of fish life, however the coral is way better in the Bahamas. We also dove off Maui for a couple of days, from what I saw Molokini backside was about the best, other than that the Hawaiian islands is not really a world class dive destination. We have not dove off Oahu yet but we'll be there next year on a return from New Guinea. I'm currently thinking about not diving while I'm there, but I'm sure we'll snorkel. While we were on Maui, snorkeling at Black Rock was as good as diving was anywhere we've been in Hawaii, although you must understand 4 days of diving is a very limited experience diving in Hawaii. I have a friend who lives on Kuai who has traveled three times to Hawaii (the Big Island) to see the Mantas and he has been skunked each time.
If I were you and you don't know if you'll be back,I'd give yourself a one day chance at seeing the Mantas, then go to Kuai and most likely your plane originates out of Oahu where the Arizona is located, stay one extra night, like you said you might not be back.
Dive Safe,
Caymaniac