I've only been diving at the Big Island once, so I'm not an expert on the diving there by any means. That being said, I like the diving better in Maui. The Big Island is the youngest of the Hawaiian islands, so there are a lot of awesome underwater lava formations (think the Lanai Cathedrals). There is also a nice spot that manta rays frequent, and I've only seen them once diving Molokini. I don't know anything about diving the eastern side of the island, as all my diving on the Big Island was done off of the Kona coast. I would assume that most of your dives would be from shore if you moved, and shore diving is probably comparable in both places. If you're going to be doing a lot of boat diving, I don't think anything can beat Molokini (especially the back wall). If you like wrecks, I don't recall there being any off of Kona but I could be mistaken. Maui at least has a few, plus a nice WWII airplane accessable to tech divers.
If you're looking for a place to move, both places have their benefits. Real estate is a little cheaper on the Big Island and there aren't as many tourists (and the accompanying traffic). The landscape is also a bit different. The Big Island is rockier while Maui has much more greenery. Personally, I find Maui a prettier place and would rather live there. I liked the Big Island (hey, it's Hawaii!), but I'd rather visit Maui if I had the choice.
Basically the big difference between diving in Maui and the Big Island is the underwater lava formations. The marine life, viz, and dive conditions are pretty similar. I'd go dive the Big Island and see for yourself, you won't be disappointed but I doubt you'll be overwhelmed either.
