I dove with Bubbles Below & Linda in 2004 and enjoyed their crew---I'd recommend them heartily and I selected them based upon reviews on Undercurrents (maybe here too). I'd skip Sheraton Caverns----I dove there 5-6 years ago and don't remember anything terribly remarkable, but maybe I just missed it. I believe that it is also quite a haul from the launch Bubbles Below uses, so they likely don't dive there much---one of the cattle boats dives there all of the time.
Bubbles Below had a couple of DMs and 8 or so divers on their boat when we dove. The DMs were good, the diving casual, and everyone dove their air, which was good since some of the folks ran through steel 100s quite quickly. If there is anything you are interested in seeing, let them know and they'll try to find it for you. The DMs were all very interested in critters---we saw a monsterous trevally, a nurse shark and a some colorful crabs on a night dive---some sort of decorated lobster as well.
Hale O'Honu (House of Turtles) is not to be missed if you enjoy diving with the turtles. The turtles are pretty friendly & will swim right up to you if you just hang in the water near them. Since they are protected, the turtles have no fear of people. One of the DMs talked wistfully about the good old days when you could pet the turtles---evidently they quite enjoy a good scratching & would swim right up to you & ask for it.
I would've loved to get to Ni'ihau, but the channel was too rough for the crossing. It is supposed to be great diving---maybe I'll get there next time.
Enjoy!