I'll have to respectfully disagree that shore diving is "better" than boat diving. Although I've done some pretty bland diving at Molokini, some of my best Maui dives have been there. I have over 200 dives in Maui and Lanai, and nowhere else have I been treated to seeing 7 types of Nudibranchs, Galapagos and grey sharks, Eagle Rays, an enormous Manta Ray, Frogfish, Hawaiian Turkeyfish, octopus, and hundreds of fish (many endemic)....all in 2 charters in the past week. And that doesn't even begin to scratch the surface of what we saw at off shore sites like Red Hill and 85 foot pinnacle these past 2 weeks.
Like everything else it just depends on the day. Heck, in the past 2 weeks I've gone shore diving at Makena Landing twice. The first time we saw turtles and a cool Frogfish....but that was IT. The second time about a week later, we saw sharks, turtles, 2 Frogfish, 2 Hawaiian Turkeyfish, Hawaiian Orange Fireworms, and about 4 different types of nudis.
Dive sites change almost daily sometimes.....whether they are boat sites or shore sites.
As far as recommending where you should go, Maui offers you the ability to dive 4 different islands and many land activities. If you come in the winter, you have the whales concentrated into the channel between the islands, providing you with great whale watching and fantastic opportunities to hear them sing underwater.
However, if you are interested in the Manta night dive, the Big Island is the place to go. Great boat and shore diving and a visit to an active volcano is always cool.