Most of this information has been posted here before, but I will summarize a little:
* Do the manta dive if at all possible
* Most ops leave from Honokohou harbor. If you want to dive up north try either Kohala Divers (I like them) in Kawaihae or Blue Wilderness (in Waikoloa). Blue Wilderness has a smaller boat that they launch from the Puako ramp.
* Hawaii diving is more about the endemic fish and inverts, and the underwater topography. There isn't a classic soft coral reef.
* The area around Honokohou is (wildly generalizing) more rubbly on the bottom. Up north the bottom is more completely covered with coral. That said, it's the rocky overhangs and caves that tend to have the nudibranchs, pipefish, sleeping turtles, etc.
* Most (all?) Big Island sites are within a reasonable swim from shore. Not all of them have accessible entries.
If I had only one day to dive the Big Island, I would dive Puako.