I've snorkeled all over Maui/Hawaii. TBH, despite what most of the guide books say, I liked the big island the best. Pristine snorkeling without the crowds, even the places described in the guidebooks were usually pretty deserted. Because there are less crowds, there tends to be more fish and more coral since people aren't stepping on it all the time. The only major drawback is if you leave your car somewhere alone, you are almost guaranteed to have your car broken into - looking for piles of shattered windows on the ground is a good way to determine the safety of you car (better yet, just leave it empty a window rolled down and the trunk open).
To me, Maui was over hyped. All of the spots were packed with tourists (I saw kids harassing turtles wherever I went) and I wasn't very impressed by the coral. The only place I went that really impressed me was all the little coves around La Perouse Bay. Not so great on coral but the fish were incredible and it was mostly deserted. Molokini was underwhelming, may have just been an off day when I went.
-Tre