We usually visit Hawaii once or twice a year; I'd like to go more but things don't always work out. You didn't say what time of year you're thinking about going. That will affect a lot of things, because it might be raining or stormy, or clear, sunny and calm conditions. There are a lot more dive options on Maui than Oahu from what I've seen
Maui is great. South at the Kihei to Wailea area is usually dry, and Molokini crater is the usual boat dive for the first dive of a two tank trip, then something close to shore for the second dive (before the wind / seas pick up). I've had great dives with Pauline from Mike Severns Diving and use them every time. Up from Lahaina and Kapalua is often windier, cooler and a bit wetter, and dive trips usually go to Lanai. Extended Horizons is great. Maui Dreams in Kihei is a good choice if you want to do scooter dives from shore, but they will go with you
Condos give you more flexibility than a hotel - you get a kitchen so don't have to eat out 100% of the time (it's nice to pick up fresh fish and grill it, then stagger back to your bed without getting in the car). The rental market in Maui is somewhat depressed due to the economy (ask me how I know). Lots of great deals on VRBO, or ask people you trust about where they stayed and what they thought. Kihei rental condos are relatively economical and convenient to many activities. You should decide if you really want to stay in the same single unit as all your relatives, or rent several condos in the same complex and have space to yourselves, since it's supposed to be a vacation. Make sure you know what you get from VRBO, since you might not get all the local help and support you get by renting from a condo complex with an on-site management rental office (i.e., getting something fixed if it breaks)
A good friend from MD goes to the big island several times a year (his wife retired from United, so flight cost isn't an issue), and he loves the diving there more than Maui. But all he does is dive, eat and sleep. We're going to overlap with him later this week and I'll post what I think after not being on Hawaii in over 6 years
