I've been to Hawaii a bunch as a diver, mostly diving the Big Island, Maui, and a little on Kauai. Only one dive trip to Oahu. I mostly shore dive and prefer the BI over Maui by quite a bit - IMO clearer deeper water, more and bigger fish, and better and more shore dive sites. Most shore dives on Maui are going to be shallow, many no more than 15-20 ft. Kauai is a distant third from shore. Having said that, for your first trip you can find plenty from shore from either island, and you might prefer the sandy beach shore entries on Maui to the lava bench entries on the BI. I prefer the bench shoreline but either can be dangerous and absolutely demand a sober eye (there are a couple of spots on either island with generally reliably tame entries). It might be that beach entries are just more obvious when they are no-go (but also tougher per se IMO).
Boat diving is a toss up IMO. Figure $85+/tank before gear. Maui has Molokini and possible hammerhead diving at Kahoolawe, which the BI can't match. BI has pretty reliable manta diving, and more lava tubes. Maui has more beaches by far. Topside attractions local and on the rainy side I rate about the same, possible edge to Maui. General layout - where you can stay on a "budget" and what it's like around where you stay = BI.
You need a car if you want to do much away from your lodging. For lodging I prefer the condo district running south from Kailua on the BI. From several you can shore dive right from the property. For hotel lodging, Maui probably has better situated hotels, with large enclaves both north above Lahaina, and south in the Wailea district. Most on the BI are isolated artificial oases up north on the windswept barren lava fields of Kohala. Some good diving and dive support up there even so.
Plenty of posts in this forum regarding dive ops.