When the vis is good, each of the Hawaiian Islands has spectacular diving. They are all different though so it's like comparing apples and oranges.
In the Summer, Oahu's North Shore has some of the best shore cavern/lava tube diving at Shark's Cove/Three Tables. On the West and South sides there are impressive wrecks to dive nearly year round; Mahi, Sea Tiger, YO-257/San Pedro and Corsair. There are quite a few other good shore and boat dives when visiting Oahu.
Maui County has some of the best diving in the US, but most of it is off the other Islands (Islets); Molokini, Lanai (Cathederals), Molokai (Moku Ho'oniki Rock, rarely dove). There is plenty of Maui shore diving but Developement Fever has had a negative effect on water quality and visibility. On the West Side, Mala Wharf is nearly guaranteed white tip reef sharks, Black Rock is a renouned beginner dive and Honolua Bay (Summer Only) is very nice for those that don't mind a long walk and long surface swim. The South shore has Ulua, Makena Landing/5-Caves, Ahihi as well as others. There are also a few wrecks, St. Anthony, Tofuda (rarely) and the recent Carthaginian II.
Comparing days with the best vis, Oahu's wrecks and North Shore (May-September) are way better than Maui's wrecks and shore dives. For non-wreck boat dives, Molokini, Lanai and Molokai are the best. The Hawaiian Islands are rarely dive only destinations with time of year and non-diving activities having a big impact.
Where Maui wins is with Manta Rays, Spotted Eagle Rays, Whale Sharks, Reef Sharks and Humpback Whales (November-March). You are more likely to see more of these creatures when you visit Maui.
I choose to live on Maui because my overall lifestyle is better, but I miss Oahu's North Shore, wrecks and night life.