I just returned from Maui today and stayed at the Grand Wailea, which is amazing by the way, so I will try to give you my perspective. 3mm full wetsuit was fine for me, water was 71 - 75. Hearing the whales underwater was amazing as well.
I went diving two days with Scuba Shack. We hit Molokini inside and outside, 5 caves, and Red Hill. Saw everything from some reef sharks, to a million sea turtles, anglers, leaf fish, etc. I will try and upload pictures this week. I had a great time with scuba shack, and if you don't own your own gear they are the cheapest. If your booking two days it will be $120 per day, with all equipment rental, including dive computer, included. You won't beat that. I'd say they are more geared towards the experienced divers because they aren't suffocating. They are safe, but they expect you to know what you are doing. If someone is sucking down air, they go up and do their safety stop above the group and then return to the boat, it doesn't end the dive for everyone else. Jim, the younger guy, was great at finding all the little nudibranches and critters that you would probably just pass over. Two things seem to bug people about them though: 1. They tell you not to pee in their(or your own) wetsuit, and 2. On the way back to harbor, you get a quick 4 minute pitch about the used gear for sale and about the owners rental properties. It was really nothing, and I can't believe how sensitive some people are.
Now obviously I have very limited experience to be suggesting an outfit to go with, but if you look, everyone has there own choice they are comfortable with, but I really think there is little difference among them all, so I say go the most convenient and cheapest.
As for other things to do, I went snorkeling off the rocks to the left and right of the Grand Wailea's beach to supplement my itch for diving. Tons of coral, turtles, and fish to see right over there. We also did whale watching and the crater, both awesome. Some awesome photos including about 6 breaches and one monster fully out of the water.
That's all for now, I'll let you digest that and if you have any other questions let me know!