I don't know about Kauai as I've never been there, but I just returned from 4 days on Maui.
If you will be staying in the southwest part of Maui (Kihei or Wailea) I highly recommend B&B Scuba, as have many others on this board. Brad and Blesi are very nice individuals, and lead nice dives. While their boat is small I didn't feel all that crowded, as everybody on my group was cooperative about sharing space and staying out of each others' way.
However, B&B expect you to be at Kihei boat ramp around 5:45 AM, and if you're staying in the northwest part of Maui (Lahaina and especially Kaanapali) you will be getting up VERY early in the morning in order to get there on time. Leaving my Lahaina hotel at 5AM I got there around 5:40 AM; I would imagine the Kaanapali folks have to leave a bit earlier. For this reason I don't necessarily prefer B&B over Lahaina-based dive ops for those staying in northwest Maui. B&B's advantage (as is that of any other Kihei-based operator) is that they are physically closer to Molokini, if that's where you prefer to go. Kihei to Molokini on B&B's boat was something like 15-20 minutes.
I also used Lahaina Divers and while yes you could classify them as a "cattle boat" (there were 20 divers on my group, plus 4 dive leaders) the boat is sufficiently large enough that this wasn't a problem. Plus they were only a few minutes' walk from my hotel and I didn't need to be there until 6:30 AM. For those of us who are not "morning people" that extra sleep is well worth it. The disadvantage for Molokini-bound divers is that it takes nearly an hour for them to get there. I understand that for Lanai-bound divers it's better to use Lahaina Divers (or another Lahaina-based op) over a Kihei-based one as they are closer to that island.
Both were equally adept at servicing new and inexperienced divers like myself. When I was there, both boats had a couple of students on board.
In addition to location, another thing you may want to ask about is location of the dives on the days you wish to dive. If you are doing your "checkout" dives you might not have any say as to location (though from what I experienced, the inside of Molokini should be easy enough for student divers) but if you decide to continue diving in Hawaii after you get your paper c-card, you may find that your favorite dive op may not go to a place you were hoping to on a certain day. For example, Lahaina Divers only goes to Molokini on Tuesday, Thursday, and Saturday mornings, and to Lanai only on the remaining mornings. They also have another charter to the back wall of Molokini (considered an advanced dive) on Wednesday and Friday mornings. Afternoons are spent at one of two Turtle reef locations. They also do both dives at one spot. OTOH B&B Scuba does its first dive at Molokini, and a second dive off the southwest shore (we dove off "Red Hill").
Have a great time on your dives.
